I've been having this problem only recently, maybe the past two weeks. I noticed it one day sitting in my driveway with the car idling. Occasionally the exhaust would make a really deep tone, and the RPMs would jump up almost to 2K. It stays at around 800 normally, but just jumps up out of nowhere. Now I'm starting to notice it while I'm driving. Tonight, after accelerating down a street, I was slowing down with my foot on the brake, and the RPMs just jumped up to about 2500 and the exhaust rumbled again. Then, sometimes after I accelerate I let my foot off the gas and the RPMs stay up there, then about 2 seconds later it goes down and the car jerks a bit. For some reason I have a feeling it's a fuel delivery problem. I don't have a CEL, and I replaced my fuel filter with a FRAM one about 5,000 miles ago. Never had to replace the fuel pump ('99 ATX w/ 62,000 miles).