In 1989 626 non-turbo with 102,500 miles, I've had problems recently with idle holding at 1500 rpm. Sometimes idles perfectly at 650 rpm and runs well. Another start, another day and for some hours will idle at 1500, sound slightly "thick" (not rough) at 3000 rpm on highway. Milage drops from usual 26 avg to about 20 avg city/hwy. Excellent mechanic cleaned idle switch etc. But unpredictable problem persists. O2 sensor? Mass air sensor? Other? Any thoughts?