Transmission Problems?

1. 1999 Ford Contour
2. 103,000
3. Automatic Transmission

My contour is turned out to be a p.O.S. About 5 months ago I went to a mechanic with "jolting" problems which I attributed to the transmission struggling to shift gears when accelerating in D. The mechanic had me replace my speed sensor and flush the transmission, but to no avail. I had come in a few weeks later (check engine light, and flashing o/d light) They adjusted the clip in the sensor and said to come back if anymore problems occured.

I am confused because it has still been acting up and when I finally get in they said they would replace the sensor and I need to have my belt and tensioner replaced and drop another $300.

I just want a car that shifts and drives smoothly, doesn't sound like its dying from such ominious and terrifying noises, and doesn't delay when I try to accelerete or jolt violently.

PLEASE ADVISE!
THanks