The pressure reducing valve was sticking and not compensating for the pressure drop when you turned the faucets on in the house. The pressure would fall off from 80PSI down to about 10 before it un-stuck, then it would go up to 80.
This repair was definitely not as easy as it could have been. The collar coupling could have come off the inlet side (top) a lot easier. Then I'd have been able to slide the valve over and twist it off the pipe end that is heading toward the house. I could have then twisted on the exact replacement valve I located at a local plumbing store.
Unfortunately, while I was working on the collar coupling, I was backing with a wrench on the valve body. The backing wrench started to slip, but I thought it was just unwinding off the outlet coupling and I ignored it. Instead it was tightening, and twisting the tubing. I had to cut it out and go get another section of copper tubing.

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