04 GMC Sierra 305 (5.0) pops, farts and wheezes and dies under load...
 

I have a 94 Z71 (I purchased new in 94) with a 5.0 305 throttle body and occasionally, when on the interstate and under load it begins to chug and back-fire and stalls. I can turn off the key and restart it and it seems to run fine for a bit and then does it again...other times.,after re-starting, it runs fine for days. It seems to occur when under load. The first time this occurred I was headed on vacation with my boat in tow and a golf cart in the bed and she didn't like it and started bucking, hissing and popping like a mad mule. I took it home and left it.

Upon return, after unloading it, it ran fine and the problem didn't re-occur for a year. Now it's happening more frequently, once a week or so and generally when going up a slight grade on the interstate (we live in Florida).

The gas mileage is poor and power seems low when accelerating. I don't want to start merely replacing parts and start with the best guess. I'm wondering if it's the ECM since merely shutting it off and restarting seems to reset and it operates until the next time it happens. It doesn't seem to do it at low speed while driving around town but when a load is incurred although the gas mileage is poor.
The plugs and wires have been replaced in the past 2,000 miles.

I'm leaning to the ECM due to the ability to turn off and restart like nothing ever happened.

Any suggestions would be appreciated.