RPM lowers-raises-lowers-raises up decelleration
 

My 91 Camry was running great until I changed the air filter! Since then, when I'm driving along at ANY SPEED, when I let off the gas the slightest bit, the engines sounds like it's going to die, then goes back up in rpm, then down, then up, several times in sequence. The engine never does actually die, just goes up and down in rpm whenever I let off the gas. At first I thought maybe I caused an air leak when I replaced the air filter but I checked and can't find anything I might have messed up.

Does it sound like I may have accidently unplugged something? Or maybe some kind of sensor is going bad?

In case anyone might ask: No, the check engine light has never come on. It starts fine, idles fine, and runs fine. The only issue is this lowering/raising/fluctuating engine rpm only when I let off the gas. It's the same regardless of the engine being cold or warm.

Would a diagnostics code self-test show any codes even if the check engine light never came on?

Any advice or help is greatly appreciated. This sympton is VERY annoying! When I got the car last month it never did this. Like I mentioned, it only started after I replaced the air filter. I also have replaced the cap/rotor/wires/plugs but it still did not do this until a week later when I replaced the air filter.

Hope I've given enough info for a possible idea of what it's doing and how to remedy the problem or what to check.

Thanks so much!