Poor Dash AC Operation in 2011 Phaeton

The in-dash A/C performance in our 2011 Phaeton had degraded since last time we used it, about 6 months ago. Having believed it was low on coolant, I took it to a shop to have it checked. The tech said there was a slow leak from the high side fitting which he changed and recharged the system. When I went by to check on status he said I had other problems. He said the high side pressure was near 400 psi but had not isolated the problem. I took the coach home for the week-end with plan to take it back for further work the subsequent Monday. I checked the A/C and found that it would provide cool air for 2 to 3 minutes after A/C start and then start failing. At A/C start-up the high side line would quickly get hot and the low side would get cold and then start warming up. The sight glass which started clear would get frothy as the cooling started and then get clear as the low side started to warm up and A/C air warmed up. The High side line stayed very hot through the whole process.

I would appreciate any thoughts as to what the problem and solution might be.