High and low Idle

Six cylinder all wheel drive automatic 90,000 miles.

My vehicles engine is starting to idle high, sometimes, and to idle low, sometimes. Sometimes while driving the engine will accelerate without gas and upon putting in park the rpm's will rise to 3500-4000. Sometime while driving the idle will be low, 400 rpm's at stop and sometimes stall. Had mechanics look at problem but no advice. Had fuel filter changed, seemed to work for a couple weeks, but the problem still persists. Tried oil treatment, gas treatment and higher octane fuel. Any Ideas/Solutions?