My customer has a air cooled 10 ton portable chiller. r-22, copeland scroll compressor, sh-12, sc-17,. ls psi 90, hs psi 340, ambient is 95. Unit is consistently shutting down on high head. We have manufactured this chiller the same way for many years. in fact the customer has two other chillers of ours, both 10 ton and operating properly with normal pressures. One thing that is odd, we tripped the head pressure switch by blocking off the condenser the one day. On the next restart all our pressures were fine and the system ran well all day. the next day it was right back to it's tricks again, shutting down on high head. Anyone have any ideas?