buick transmission locks

My 95 buick regal transmission locks and the car won't move. when I
start the car it runs fine, then when I shift to any gear the engine
labours but the car doesn't move. If I shift from R to N to D and back
a couple of times with the engine reved to about 3000rpm I can get the
transmission to unlock and operate properly (no slipping, long or hard
shifts). If I leave the car in neutral and turn the key off, then
shift to park and remove the key, I can insert the key and shift to
neutral to start and drive normally. If I shift to Park then turn the
key off or start the car in park the transmission locks again. What's
wrong??