Lincoln Continental -small one,Trans Question on over drive?

The trans jumps down out of overdrive while at speed and them at some point goes back in.After changing the valve body and solenoid pack it still does it,Itseems to do it at a lower RPM.I was thinking it could be a grounding problem,speed sensor,or computor quad driver failure intermitant to trans solenoid pack.What do you think is the problem?