2001 Saturn L300 RPM drop off during idle

In the past couple of months, I've noticed that the engine RPMs fall off as follows: falls from 650 down to around 575-600 range, is a momentary drop after which the engine recovers (but sometimes feels like it wants to die), most often noticed when the engine is thoroughly warmed up or hot, and doesn't seem to be a problem during normal driving speeds (only happens when I'm stopped). I've replaced the plugs, checked the plug wires, replaced the air filter, cleaned the throttle body, fixed a leaky vacuum hose and inspected the others, tried gas treatment, disconnected the battery to reset the computer, checked for trouble code (none recorded), replaced the gas cap (had a one-time vent valve error code, cleared it and no repeat in 1000 miles), and checked around the gas tank for signs of leaks (none). Fuel pump and level sensor are less than 1 year old (old pump died, changed both while the tank was dropped). Don't know if it's related, but also recently have twice had trouble starting after filling the tank (engine turns over normally but doesn't catch until the second try). The second time it happened I pulled away from the gas pump to the other end of the lot and tried starting and restarting the engine several times and noticed that it didn't shut down cleanly but instead had post-ignition knock for a second. Problem cleared itself up and hasn't repeated but it's been less than 200 miles. Engine still has original sensors and attachments, except for crankshaft position sensor (which was replaced about a year ago) and the usual consumables (plugs, air filter, oil & oil filter at 3K intervals). Only real problems are a small coolant leak (oil cooler section), A/C hose is bad, and oil in the spark plug wells (on my list to fix when $$$ comes available, maybe sooner if that's the source of my problem). I rack up about 100 miles/day in city traffic and these is the only glitches I've noticed but it's got me beat! Any help would be greatly appreciated, thanks!