Chevy & GMC Vans

I have had this problem on and off since I bought my van. When you start it, the idle keeps dropping then going back up to normal. It usually stops after a couple minutes, but sometimes it will do it even when it's warmed up. It occasionally has a miss while idling/ driving. I have not noticed a major loss of power. It also will stall when going from Park to gear. It has done this 4 times, but it only started a week ago. It always immediately refired. It also will not run in the rain (doesn't like to start nor stay running).
I recently took it in for the issue. A known good idle control module was installed , which did not help nor hurt the issue. My mechanic said it has proper fuel pressure and the coil seems ok. He said, as I have, that it would not always act up. He said that the computer and engine may not match (computer was replaced prior to purchase of van, but he did not know that).

This issue is driving me nuts. Can anyone help me out? I don't want to just throw parts at it and hope I fix it, as that won't be cheap.

A new set of plugs, wires and AC Delco cap and rotor will help...unless they have been changed. My 95 had a weird idle as the 92 - 95 "W" motors are known for. It never stalled though. Not starting in the rain is a clue to an issue with ignition. If you can, remove the doghouse when it's wet out and see if there are any arcing or sparking going on around the ignition parts.