i have a 98 ford contour se v6, with the same problem. I noticed the 1st time it happened i thought my tranny was going to fall out. i pulled over and turned off the car and sat there for a few wondering if i should drive it any further.

when i started the car back up and took off the shifting was normal no more problem, until i jumped on the freeway again going about 60-75mph. got off the freeway came to a light. took off and the shifting was rough again.

it seems like it could be a sensor or electrical problem.
AAmco told me i needed to replace the tranny. 2500.00 - 3500.00 dollars. i don't think so.

if you have fixed your problem can you help me.
thank you :