TCC oscillation with no PCS readout

I put a used transmission into my car last week and have noticed that on the highway or anytime the torque converter is supposed to be locked, I get an oscillating lock/unlock at 10-15% throttle. More throttle keeps it unlocked, and less lets it stay locked.

Upon scanning, I noticed that my PCS line pressure was frozen at 38%. Could I have a bad connection at the plug, or could the PCS be completely dead?

BTW, donor car supposedly drove perfectly fine, but blew head gasket and overheated (I helped tear the engine down). This was over a year ago.

Still having issues. Anyone with a similar problem or some advice on something to try?

Got it figured out! I did a little more reading and found Russ K's instructions on gear changes. I used his method, and everything is working flawlessly. Thanks, Russ, for sharing all of your knowledge with us.