I have been having that problem with my hi miles 300 as well. Mine is a '90 model, which will be the same for this purpose. On the right hand side of the intake, the side w/ the throttle linkage on it, there is a smaller cylindrical part right on top of where the air intake from the air cleaner goes in. This is the idel/low speed air valve, and on mine this valve keeps getting clogged up with gunky oil from blow-by, causing it to run rough, lope and sometimes stall when it is just sitting idling. Runs normal otherwise. I took mine off, cleaned it, and it runs fine now. I know that this is common on these engines with high miles.