Chevrolet Forum - Chevy Enthusiasts Forums - Undiagnosable misfiring issue

I have a left-bank only misfire. It started after having head gaskets replaced. The truck is a '98 k1500 with a 350. I will give a chronology week-by-week with observations. If anyone could help pinpoint where the problem may be, that would be awesome.

Week 0: (Mid-November 2012): Head gasket on drivers side bank suddenly blew. The truck was towed to a shop. Both heads were removed and machined and both banks received new head gaskets. In addition, the distributor was replaced, intake gasket was replaced, new spark plugs and wires were installed, and PCV valve was replaced.

Week 1: Received the truck back. Test drove it before paying the bill and taking it home. The check engine light came on as soon as the motor fired up. I scanned it with my cheap scanner, and the code was P0345 - Camshaft Position Sensor A Circuit Malfunction - Bank 2. The mechanic pulled the truck into the shop, reset the light, and said that the CEL light is often on after the motor has been worked on, and the PCM just needs to get used to it, and if the light doesn't turn off in a few days, bring it back.
The light turned back on within the first few minutes of driving again. I brought the truck back, he looked at the code, reset the light, and said the same thing.

Week 2: The check engine light, at this point was still on. Because I work back home on weekends, and am away at school on weekdays, it was inconvenient for me to bring the truck to town when the shop was open. The light stayed on, and although we called the mechanic to report it to him, he said not to worry about it.

Week 3: While sitting at a stoplight, the motor started to misfire - quite violently. It felt as if it was about to stall. The motor would have a steady streak of misfires for about a second, then run properly for about half a second, then continue misfiring for a couple seconds, then running properly for a fraction of a second, etc. etc. etc. etc. etc. I scanned for codes and got P0345, P1345, and p0300. A few days after this, the P0345 and P1345 went away and haven't come back.

Week 4-5: I was essentially forced to limp along with the near-constant misfire. At highway speeds, the TC would remain unlocked, but you could feel the engine hesitating often, even on level road. Often, when the truck would start to misfire, the CEL would flash, indicating that a lot of unburned fuel was going out the exhaust. I would get codes P0300 and P0305.We were able to set up a weekend appointment with the mechanic. He discovered that his timing, when he installed the distributor, was 20 degrees off. He adjusted the timing with his computer till it was perfect, and assured me it would be fine now. He also looked at the misfire history. The most misfired had occurred in cylinders 1 and 5, with just a few in 3 and 7. The misfire count in cylinders 1 and 5 was increasing as the truck was just sitting there idling. None of the right bank cylinders had even a single misfire in their history. I drove away, and within about 5 minutes, I could feel misfiring again.

Week 6-8: Went back to the shop, he said that the only thing that could be wrong was plugs, wires, or the distributor, however, gave no offer of helping and, in fact, denied that he had installed a new distributor cap. I went through to make sure wires were snug, and gapped and replaced all spark plugs. This did not help.

Week 9: Brought the truck to an auto electric shop. They tested all of the plug wires and said that all the plugs were getting a hot spark. They cleaned up and tested the injectors, so they are all working. They did compression tests and reported that the compression in the engine was "phenominal." By this time, the severity of the misfires had dropped, although they were still occasionally happening.

Week 10-12: Seafoamed the heck out of the engine. For about 2 weeks, it ran great. There was the occasional gentle bucking when climbing a hill, but nothing major and nothing while under light load.

Week 13: Misfire came back in full swing, after being on the road for about 50 miles. This misfire situation lasted only a few days, then went back to behaving for the most part.

Two weeks ago: PCV valve replaced. Not even a hint of misfiring, under any load conditions.

Today: Misfire came back in full swing, after about 75 miles on the road, in the form of gentle bucking while climbing a hill. Then, misfiring got to the point where it was once again continuously misfiring on just one or two cylinders for a couple seconds at a time - and the TC stayed unlocked for about 90% of the time after that, for the next 20 miles.

Observations: Misfire is not dependent on ambient temperature. Engine had zero misfire history before the head gasket blew and was replaced. Misfiring is in multiple cylinders in the drivers-side bank, and there has been no misfiring in the passenger-side bank. Even when the engine behaves well, my gas mileage has dropped 10-15% from what it was before the head gasket job.

Driving habits: I drive gently, I usually keep my speeds between 50-55 mph. In my normal driving, the TC does not unlock. (Or used to not, until quantity of misfires gets too high).

OBD-II observations and ranges, today, at idle:
Short Term Fuel Trim
Bank 1: 0 - +1%
Bank 3: 0 - -31% typically, but jumping between -23 and +29
Bank 2: 0 - +3%
Bank 4: 0 - +42%, typically about +12%

Long term Fuel Trim
Bank 1: 28%
Bank 3: 75%
Bank 2: +17%
Bank 4: -48%

Manifold Absolute Pressure 31 kPa

Timing advance 21-23 degrees

Short Term Fuel Trim
B1S1: 0.0%
B1S2: 99%
B2S1: 0.0%
B2S2: 99%

O2 Sensor
B1S1: 0.0 - 0.8V
B1S2: 0.0V
B2S1: 0.0 - 0.8V
B2S2: 0.2 - 0.4V

Sorry for the really lengthy commentary. If anyone can help, please do. I love that truck, but I'm at my wits end with the misfiring.

Thanks all