93 Bonne SE na surges at idle
The engine on my 1993 Bonneville SE surges at idle. It idles fine some times, then for no good reason revs up to 1500 tp 1800. Also having trouble getting the tranny to stay in overdrive. It wants to downshift without being prompted by the throttle then it won't go back into OD until you turn off the engine and restart it. Both problems appeared at the same time, not sure if they are related.
Checked for vacuum leaks, seems fine, steady at 20 inches vacuum. Replaced the Idle air control this week. The TPS is about 2 yrs old. I am getting codes 13 ( 02 sensor), 21 (TPS high voltage), 34 (MAF), 39 (TCC), 42 (electronic spark timing)
Looking for sugestions of what to try next. Thanks