1997 Mercury Tracer 4 cyl Two Wheel Drive Automatic 105353 miles
For the last few weeks I've noticed the car doesn't seem to be shifting as smoothly. It's taking longer for it to switch gears and while idling the engine seems to struggle to stay on it seems. Today while on a long trip the speedometer started jumping/floating. I'd been doing about 70 on an interstate for about 15 mins when the needle jumped down to 55ish then back up to 70. I slowed down and it would repeat the issue. It seems to jump down approximate 10-15 mph in reading but the car is going the same rate of speed. I put the cruise on and after a few mins the needle dropped briefly and the cruise shut off. It appears to be quite random and since we stopped for a break (left the engine running) it hasn't repeated. The shifter was in the proper place when the problem occurred, was the first thing I checked after turning off the vent, lights, and stereo. Any input would be very appreciated.
When engine power drops, the shifting would be affected. Since the idling is struggling, you need to get it checked. Could be the spark plugs are due for replacement or other parts related to the sparks.
Another fault seems to be the speed sensor. It is most probably going out and when the speed signal is incorrect, shifting would be affected as well.