i still have this problem from about a year ago but is getting worse, i have a 97 chevy pickup c1500 and it is a 5 speed manual with the 4.3 v6. It only did this when it was wet out when i first got it.. when im accelerating going through the gears its running smooth but when im at a cruising speed and i let up on the gas a bit it starts missing, or if say im in maybe 2nd gear and i cruise up to about 20 mph and the rpm's are at 2,000 and i leave it there letting up off the gas a bit to steady it at 2,000 it will start missing again. this is anytime, but if it is raining out the truck will start up but struggle to move and will start miss firing no matter what all the way through the gears,and it will deffinately start missing if i go through a puddle because something is getting wet maybe? causing the missfire? but when its dry it will only do it maybe around 1500 rpm's but only at a certain speed like 30mph. I am so stumped on this one i have all ready given it a tuneup, changed the timing chain, and replaced the gaskets on the upper half of the motor. I REALLY need some help with this one guys. and this isnt just a normal "barelly notice missfire" its a hole truck jerking back and forth shaking the crap out of you and how is this thing running like this kind of missfire.

 

Got on this evening to post this exact problem.
Hope someone has some good ideas.
David

 

That pretty hard to figure out, are there any vacum leaks anywhere, does the year have any vac advance on the distributor?

 

did you pull the distributor cap off and check under it?
a bent valve or worn cam lobe sound possible.