Hyundai Repair: Intermittent Overdrive problem, transmission control module tcm, transmission control module

Question
I have a 2000 Tiburon with automatic tranny, and the Overdrive
button on the side of the shifter. About once a week, it
fails to shift into overdrive (button is in correct position),
when going over 45 mph. If I shut off the car for a minute,
then start it back up, it fixes itself. I'ts been doing this
for over 6 months now. The dealer of course wants to replace
the tranny for big $$. It shows the P0734 code, but no chk
engine light ever comes on. I've heard the wires at the
bottom of the shifter can break over time and cause this.
Any other ideas?  

Answer
Since the problem goes away when your turn the vehicle off and restart it, it's my presumption that your transmission control module (TCM) is commanding failsafe (3rd gear only).  The P0734 code indicates that the transmission is slipping in fourth gear and is consistent with that.  Unfortunately, the dealer is probably correct.

The overdrive button operates by sending power either to the O/D OFF lamp in the cluster or to the TCM.  When the TCM receives power on the O/D switch circuit, it enables overdrive.  Just because the O/D OFF lamp is not illuminated, it does not mean that the TCM is receiving the voltage on the switch circuit.  There could be a broken wire or a defective switch causing the TCM to not receive the signal.  But I wouldn't expect restarting the vehicle to affect whether overdrive is enabled if this were the problem.