Non-shifting Transmission Overdrive Issue

hey there

3.8 liter silohuette - 95 - It will not stay in overdrive at high speeds. No other problems shifting.. When driving at high speeds the trans slips out of OD and shifting manually does nothing. Trans has been rebuilt recently and the trans shop can't figure it out. 135,000 miles on vehicle. Been dealing with this for a while and its time for a road trip and it needs fixing...any info would be very helpful and appreciated.

thanks alot

pat