I can almost guarantee it is the TM751 going bad.
I have two older Date Code: 00D17 TM751 modules.
I did a test with my test rig powered through an XPPF filter so my XTB-IIR probably didn't get into the mix.
Older LM465 Lamp Module and the TM751. Controlled by a HR12A Palm pad.
Address B1 so both the TM751 and Lamp Module would be controlled on the same address.
One acts normally but the second one with a possible power supply problem did not.
When it warms up it does basically what you are seeing. Sometimes hitting the blue dim from an off. Will set the lamp to 100% as the older modules should do but sometimes it would go to 100% and then dim all the way down to 0%. At that time I could not toggle the internal relay in the TM751 on and off. It was locked up and a power cycle sometimes corrected it. Sometimes it had to cool off first.
I added my ELK ESM-1 to the setup and the bad TM751 was sending a continuous 10 bar stream of X10 signals when the dim acted strange. Sometimes it went to 5 jumping bars of noise as the X10 LED went off. Guess that is why it was getting warmer than normal. Power line transmitter on 100% of the time until power cycled.