4T60 no forward or reverse after sitting overnight

1991 Regal Custom 3.1 78,000 miles. Car sat unused for about 5 years. Transmission won't provide any forward or reverse movement for a while (any where from 5 to 360 seconds) after sitting overnight. This problem is somewhat intermittant as there are days it has no problems. Once it starts moving, there are no noticeable driveability problems. I have change fluid and filter, added Lucas treatment, and installed a cooler line check valve. If I pull the vacum line off the modulator to increase pressure, the tranny will go into gear almost immediately. At this mileage, does this seem like a major problem requiring rebuild or replacement?

Sounds to me like the vacuum modulator is bad and needs replaced. Not hard or expensive to do and you already know where it is located.

I purchased one this morning. I will update once it is installed. Thanks for the reply...

The vacum modulator did not help. Any other ideas?

Ordinarily the loss of DR or reverse is internal, if the fluid is full and clean, yhe trans is going to have to come apart, sorry.