[Davescort97]

Welcome to Automotive Forums. There are several conditions that could cause the tranny to not stay in OD. If it surges while cruising the torque converter clutch solenoid could be keeping the torque converter from locking up. It is an electrical solenoid so check the electrical connections around the transmission. The throttle valve control cable could be too tight causing it to not stay in OD. Loosen it 1/16th of an inch at a time until it stays in OD.
It's on top of the tranny along side the shift cable. Last, but not least, having no upshift into OD could be caused by a bad one-way clutch inside the tranny housing. I think there's not enough slack in the throttle valve control cable causing it to jump in and out of OD. This is what I"d check first. You always want to try the easiest things first. Dave