2000 Rx300 Rpm Moves Slightly In Park, Neutral Or Stopped At Lights

[natesniper8]

I have a 2000 rx300 with 131,xxx miles. This first started on a 18 hr road trip. When we got the the rpm moved up and down slightly but noticeable at stop lights or slow speeds. Someone did fill it with octane 85 when i was sleeping during a fill up or 2. After we let the car sit after we reached destination the problem was gone. I returned back home to MN and there were no problems. I'm not sure it this is connected to the current problem but it could be.

A couple weeks down the road the rpm started to move when I first started the car and continued at slow speed, stop lights. It was real slight and not too noticeable. I noticed it goes away once I hit highway speeds of 60-70mph for a few seconds and then returned to slow speeds/stop lights. Otherwise it continues for all in city trips since i can never reach that higher speed. At one point the check engine light went on and it read p0100 i believe and multiple random misfires. But that light eventually went away a day or 2 later, or my buddy reset it im not sure.

I read another guy on this site had the same issue months back (SRX) and a couple others. I guess motor mounts are likely to cause the problem. But that doesn't make sense to me if it goes away once I hit 60mph. Clogged injectors? I guess one of the motor mounts is hydraulic/oil.. possibly ignition coil? Any help is greatly appreciated, thanks.

Nate