Erratic Idle

Listers, I can't seem to find this addressed previously, so here goes. 88 3.2 Coupe, 97k miles. When stopped at lights/signs car will at first idle dead on at 800 rpms, then in about 5-10 seconds idle will momentarily drop and then in 3-4 second cycles with each drop getting worse (lower rpms). Possibly related...when driving in heavy rain car will sporadically run very poorly then instantly improve for a bit, then go bad. Could someone tell me the most likely things to start checking/replacing?
Thanks,
Buck
88 coupe

It happened to me 2 years ago to my '87. I'd suggest to check all the vaccum first. Make sure no leak. Next, check distributor cap, rotor and all ignition wires and plugs. At last, check the air flow meter. (the "thing" next to air filter box) If you have a friend has a spare or whatever, replace it. My problem was the air flow meter. Cost me $300 part+labor from a mechanic. Expensive!!
Hope this help!
Andy