Ford Repair: 94 Mustang transmission, vehicle speed sensor, vehicle speed sensor vss

Question
I have a 94 3.8 Mustang automatic transmission. When I'm driving and the engine (or tranmission) has been running for 10-15 mins, or on a very hot day - my transmission will all of a sudden rev up to 3000 and I'll have to take my foot off the gas and put it in 2nd. After a while I can shift back into Drive and the problem may or may not return. The only routine is that it may be heat related. I'm hoping it's a simple problem. Can you help?
Thanks

Answer
If the transmission is not up-shifting, it could be a Vehicle Speed Sensor (VSS) problem.  The trans does not know how fast the vehicle is moving and doesn't know it should shift.  This is a relatively cheap and easy fix, about $325.  It could also be a Shift Solenoid which could involve removing the Main Control or Valve Body, this would be more expensive.  If the transmission is "Slipping", that would indicate an internal problem, and might require rebuilding or replacing the trans, that would be very costly.

I hope this helps,
Dave