Shower Mixing Valve or Tank not putting hot water through

I have a shower that can't get beyond luke warm. Sink next to it gets plenty of hot water. I have sterling fixtures throughout and the house is about 15 years old. I changed out the mixing valve, but that didn't completely solve the problem. I can finally get medium warm water if I let it run for 5 minutes, and then, every once in a while full hot water comes through (not good if you're in the shower!). Attaching 2 pictures. Do I need to replace the mixing tank/fixture behind the valve, clean something out somehow, or something else? Trying to avoid the major plumbing bill if I can....

Do you have a hot water circulation system and pump? That valve does not appear to have a "temperature limiting stop", so if everything else is proper, you should get maximum hot water when it is turned all the way to the left. BUT, a malfunctioning circulation system CAN produce your symptoms.

The piece with the word "up" on it is the balancing spool. Pull it out with needle nose pliers, and replace it.

I don't. Hot water works fine everywhere else.

Thanks for the information on the balancing spool! Will give that a try. Is it there any trick to pulling it out? (Trying to avoid a Tim the Toolman moment!).

Actually, it is the whole cartridge, and the "trick" is to have a new one on hand BEFORE you remove the old one.