GMC Repair: engine misfire, port fuel injection, throttle position sensor

Question
I have a 96 chevy 1/2T 2wd truck with the 4.3L engine, 5spd, no air conditioning, 146,000 miles.  The truck used to have a problem about not starting on rainy/damp days.  It got really bad, then one night it broke down, and gave off a random cylinder misfire code.  New cap, rotor, plugs, wires, and it ran good.  It still would not start on rainy days though.  Everybody told me to replace the distributor, so I did, as well as new cap and rotor.  While I was at it, I replaced the lower intake manifold gaskets because they leaked antifreeze consistantly(replaced upper gaskets too to be safe).  I also cleaned ALOT of carbon out of the intake and off the fuel injectors.  When I got the truck all back together, it ran, but the coil was sparking badly.  I put on a new coil and solved that, but now the truck runs very rough at RPM's above 1500.  Put another set of plugs/wires on..nothing. Sometimes the check engine light is on and it gives 3 codes.  Mass Airflow sensor low input, Throttle position sensor low input, and random cylinder misfire.  Sometimes the light is off, but the truck runs the same, rough above 1500 RPM's.  It is not a consitant misfire though, sometimes it does it, sometimes it doesn't.  It seems that the harder you are on the gas, the smoother it runs.  Under a no load acceleration it sometimes misfires, sometimes doesn't.  On the bright side though...it does start in rainy weather now.  I did not yet replace the Ignition Control Module.  Thank you for any advice or direction on this problem!

Answer
Hi,Chris

You may have a CPFI{Central Port Fuel Injection} unit that is failing,GM has issued a bulliten on this and ive replaced many,theres a new injection unit issued and its a conversion to MPFI{Multi Port Fuel Injection}.you may even be able to get the dealer to install this under warranty,the bulliten number is 1234172.
Also ive seen the PCM{powertrain control module}cause your same complaint.that would need to be replaced and reflashed from the dealer.i am leaning more towards the CPFI unit causing your problem.if you need more help let me know.Thanks


ED