2007 is250 fluctuating rpms
 

My 2007 is250 awd with 155k miles has been acting up. At a stop light the rpms go from 500 to 600 to 500 to 600 and keeps repeating this loop. Has this happened to anyone before? Btw there are no error codes or check engine lights

Try resetting the ECU first. Disconnect the negative cable for about 30 mins. The car will not start the first time around try again. When's the last time you had a tune-up? Cleaned throttle body, plugs, pcv valve, etc....

Is that a sign of carbon buildup issues?

Sounds like carbon buildup. A friend of mine has some friends who work for Lexus, and their "official test" for carbon buildup isn't too scientific (because there are no error codes). The process to test carbon buildup yourself is to rev up to 4k for 30 seconds, then rev all the way up(till it peaks), then let off the gas. If the engine sputters when the RPMs drop back down to normal then its a sign for carbon buildup. Hope that helps.

had a tune up 20k miles ago. The really weird thing is that when my ac is on my rpms don't fluctuate. The ECU reset fixed the problem for a couple of hours but then the problem came back