97 Bonneville shifting problems

I'm working on a 1997 Bonneville with some tranny issues. When car is cool, trans works fine, but when it warms up, it seems the torque conv. is not locking up in overdrive (rpms keep bouncing up and down when under slight load or incline). This sounds like a tcc soleniod problem, but no problem codes. I tried a new TPS, but no change. I think there are several components that effect the tcc soleniod operation... but where to start?????