Chevrolet Repair: Missing 5.7 Vortec, throttle position sensor, mass air flow

Question
I have a 1996 Chevy 1500 Z-71 with a 5.7 Vortec, 5 spd.transmission.  Whenever it rains or the air is very damp, i.e. heavy fog, the engine misses at normal load/speed, at about half throttle.  It runs fine at full throttle or idle.  A local mechanic ran a diagnostic on it and it came back as a misfire code.  He advised me that it was probably an ignition problem, most likely a spark plug or plug wire.  I replaced the plugs, wires, dist. cap and rotor button about 15,000 miles ago.
What should I check and how do I check it?  You can give it any amount of gas while sitting still in neutral and it doesn't do anything, just when you're actually driving...  Any help on this matter will be greatly appreciated.
                         Craig Sykes

Answer
Hi Craig,
Now this is just a guess, or wild thought, but the air intake has a sensor, mass air flow, I believe, which may be miscalculating due to the density of the air during that time.
MAP sensor maybe too. That is actually a vacuum sensor.
I wouldn't run out and buy a new one.
Also, if the throttle position sensor is adjustable on that truck, see if it is set correctly.
Van