96' Stratus Transmission issue. plz help

[ihatemycar06]

I just had my transmission replaced with a used transmission a friend of mine grabbed at a pick and pull in sacramento.

After I first got the vehicle back it drove fine. Or at least it seamed to. Then I tried to take it on the freeway and realized that it would not shift to higher gears for faster traveling.

I let the car sit and cool off for a couple hours then tried again.. It shifted fine and I was on my way.

by the time I got home and did a little city driving the car started acting up again. It would not shift into higher gears.

I was curious if any body knows maybe what the mechanism is that controls when the car shifts and puts it into the next gear.

A friend of mine said it could be the thermostat that could be the problem.

Please help me.

thx