94 Bonneville will not shift into OD when warm

I have a 94 Bonnevill in witch I have swapped the drive train of a 94
La saber. For the first 10 miles or so the tranny will shift into OD just fine, but after it gets warmed up it will no longer shifts into OD or locks up the torque converter. I hope it is something simple like the vacuum modulator it only has 47000 miles on the engine and tranny. Any help is appreciated

Update: I replaced the vacuum modulator and it made no difference.
I have gone to 3 different tranny shops and they told me 3 different theory's as to why it stops shifting.
I don't no what to do next
Please help