A4LD transmission torque converter won't stay locked up

When I get past 45 mph the torque converter locks up and then unlocks. Went to a tranny shop and they replaced the tcc sensor, same problem. Now they tell me it could be the valve body, computer, or the torque converter itself. I don't want to go on a fishing trip. Anybody ever have a similar problem and if so what fixed it. The truck is a 91 ranger 4x4 4 litre A4LD transmission.

What codes are you getting from computer?
I have a 1991 with a 3.0 but my trany wont shift out of overdrive,I had the valve body ot and did everything the book says but no help,Now the book says it might be torgue converter I hope not there $$$$.