01 Overdrive Prblem

I have a 2001 SEL with 75,000 miles. When driving at highway sppeds 60 - 70 mph, the transmission seems to shift out of overdirve and the RPM go up slightly when not under load. When this occurs there seems to be a slight shudder of the vehicle. The OD lights dones not flash or say on.

Any ideas would be appreciated.

Thanks