Shower pressure low

I have a Pegasus shower faucet that I installed about a year or so ago. It was working fine up until a few months ago. The pressure would start out fine and normal when you first turned the faucet on. After a few minutes, when the water reached the right temp, there would be an audible thump in the faucet and the pressure would significatly drop and stay that way. No change in temp, just a pressure drop.

I had low pressure right after the install due to debris in the valve body. I suspected this was the case again and cleaned out the vavle, I found a little debris, which I removed, now the pressure is even lower than before. What else could be the problem?

More crud? Are you on a well?

Nope not on a well, actually I have checked it 2 more times for crud and debris and found nothing.

Why would the pressure start normal and then at the working temp drop so significantly?

Also it worked fine for months now it almost comes to a drip, it is quite annoying.