I have a Mazda 626 (V6) with about 150K miles and a funny engine racing problem. It occurs only when I’m going highway speeds (for me, 70MPH) and the car has been running for about 15 minutes. The engine will race, then chug and RPMs will drop down again, then it will race, etc. If I slow down, it will stop racing, but often start again as soon as I put my foot back on the gas pedal. If I don’t get on the highway (running 60MPH and under), the car is completely fine.
I’ve taken it to my mechanic twice; the first time had the fuel filter replaced, the second time got new spark plugs, but it’s still doing it. Any ideas what else it might be?