There are many little gremlins that cause our engines to run inconsistently or poorly...or worse. I would suggest doing a 20k maint interval and inspection as per the shop manual. Go through all the simple stuff first, then go to the diagnostic procedure outligned in the manual... For me, it has usually been the simple things. Or a Complete no warning CATASTROPHIC failure.
a rough or inconsistent idle is not usually catastrophic so I would not be worried... too much
Already on this thread, we have everything from plug wires to carb jets and gas cap vents... It could be one of these, a combination, or something completely unrelated like an intermittant electrical component that is affected by heat or vibration.
In the shop manual is a flow chart for troubleshooting these type issues. See the quik Ref threads going to "forums" these are an invaluable resource as all manuals and parts fiches are there for the downloading...
As has been mentioned many times, the proper electrical grounding of Coils and other circuits is very important and if intermittant will make your bike act as if possesed by Wotan's mechanic... the honda facto literature does not go into too much eletrical trouble shooting other than continuity and voltage measurements. I don't think they ever mention the shake test while performing these measurements... so an intermittant physical connection might be tough to find if all you do is touch leads and look at the meter. Just like spraying stater fluid around the airbox to look for vac leaks, (gently) shake the connector blocks while the motor is running, give a little tug here and there on the grounds...
30 yrs is a long time for copper wire in hostile environments... add bi-metalic joiners and electroylisis. they may look good on the outside, while black and unconductive on the inside...
All my BS aside... plspardon...
If I gotta guess, ';-0,,, I think it's a sticking float ( float stays up, fuel is gone, idle goes down, float eventually falls, prob goes away, under speed float does not stick as it's vibrating like all get out ) That's my .02, .01 whatever it takes!