700R4 hunting for gear over 50 mph

[BB72CHEVKT]

I have had this problem since I had the transmission rebuilt 6k miles ago but thought it was because I was running 2.8? rear end gears. I took the burb to a different shop and they confirmed I had 3.73 rear gears. I was thinking that once the gears were swapped to 3.73 the gearing would be right and solve the problem but apparently I had 3.73's the whole time. It does have a lock up kit installed. I am going to try to take it to the tranny shop next week but was wonding what you guys think.