502mag won't start in idle, after warm-up can be backed down to 700rpm but stalls in idle position

[Controlled Burn420]

I have a 1996 502mag, the engine has been bulletproof for the 443 (fresh water) hours and 223 I have put on it. Only one issue, occasional idle fluctuation until now. This is the first weekend I have taken the boat out and it started fine, ran fine and idled fine for abour 5 minutes on the hose. I warmed the engine up and let it run at about 1800 rpms on the hose. After backing down the throttle to idle position, the engine wouldnt idle and stalled. Now it won't start unless the throttle is about 3 clicks off idle, I can back it down and get it to run in the 800 RPM range after warm-up but it seems to be a little rough. I also want to add that I messed with a few controls 2 seasons ago, the trottle linkage, and the little set screw under the air intake. Neither one of them seemed to do anything when I was attempting to adjust them. From what I remember I think I left them near their original position. After a little more investigation (6/23/13) I found that the adjustments I made would have not affected anything. Do you folks think this is an IAC , MAP, ETS, or something else. Should I just find someone to scan prior to replacing parts? Please help, I'm in over my head and wallet!

James
San Diego, Ca.