I have a ’96 Mercury Mystique with a 4 cylinder Zetec and automatic tranny. It has about 112,000 miles – mostly on the highway. At 110,000 miles the timing belt, tensioner, water pump, spark plugs, and plug wires were replaced.

After that recent maintenance the car ran fine, but after 500 miles of normal driving something strange started happening during a recent road trip. While cruising at 70 mph, there was a slight hesitation or jerk that lasted for a fraction of a second. It was like taking your foot off the gas and quickly stepping down again. You could feel the quick change in power. This would happen randomly every 30 to 60 miles. But it would only happen at speed, in overdrive, and when the engine was hot. The hesitation would happen more often when restarting after the car was off for a while but then it would settle down after 10 to 15 miles of driving. Slowing down to 65 or less made the jerk less frequently and less harsh. The tach did not move up or down during these brief hesitations and the hesitation would happen only once each time.

I added less than a pint of Mercon ATF and tried a bottle of gas line deicer in case the fuel was bad. But neither helped. The fuel filter was last changed at 75,000 miles. Also, the engine stalls easily now when coming to stops because the idle rpm is low. The CEL is ON (it often is). The last time it was checked the code was for an upper oxygen sensor. The light can be turned off but usually comes on again within 1000 miles.

I haven’t been able to find any posting with these symptoms. Does anyone have an idea what is wrong?

whats wrong is the o2 sensor, they can cause tons of problems on these cars