I also have a 94 convertible and have the same problems...only I just turn the car off and resart and the problem goes away, or sometimes if i play with the gas pedal, it eventually shifts...this is after a transmission rebuild,although it did it before. I figured the rebuild would fix it but no such luck. Please let me know if you ever figure out what the problem is. I'm wondering if it has something to do with the ECU...
