Today I worked on a mcquay water source heat pump that was tripped on high head upon my arrival. Standing pressures were in an acceptable range so I reset the the unit. The unit kicked on immediately and ran fine with normal pressures and amp draws and TD across coil. After running fine for 10 minutes the head pressure spikes to past 350 and trips off on high head. Unit has a cap tube and low SH. The property manager said that the unit has never really worked properly since it was installed. I figured that there was a restriction in the water lines, but when I checked the strainers they were clean. Also acceptable delta T on the water. Could it be a bad reversing valve? Doesn't make sense how everything runs steady for more than 10 minutes and then boom, head pressure is through the roof. Any ideas?