My2001Altimafeels like its not idleling right. It goes up and down. At times weather it's hot or not it will stall out on me if I don't constantly give it gas when I am stopped somewhere. Then when I am driving, sometimes it feels like its slowing down and loosing power. I gave it a tune up and changed the gas filter thinking it might of beenclogged up. But none of that helped out. Some one suggested catalistic converter trouble?? or oxy sensor?? I really doubt it. Any suggestions??

 

crankshaft sensor

 

sounds like you have a vacuum leak or your air intake came off after the maf (throttle body>air intake hose>leak>maf>air filter. check those two out and let me know.) causing it to get incorrect air intake readings causing it to rev up and down also check your pcv valve to make sure its not stuck either.