New to this forum, and appreciate any help.

12000 BTU Anchor marine unit, manufactured 2004 according to info on the unit, placed in 2006 Larson Cabrio 290, sold by dealer new in 2008, bought used by me 2010. Boat had under 30 hours on it, as did generator, suggesting AC had little use by original owner. Worked fine for several months, then began to sporadically display HPF code with no water flow out the side. Progressively worse. Brought in very capable and knowledgable AC technician who replaced the original Cal pump with PML 500 water-cooled pump. Flushed all lines with Rydlyne. Same problem. Panel is FX1 from micro air. Reset to original defaults. Same. Lowered protection level to 2, then 1. Same problem. Intermittent though. May work fine for hours, then suddenly stop flow and show fault. Intake is clear. Thermostat seems fine in that when it works, it does stop when it reaches set point (usually 76). Is there a sensor ro replace? Any suggestions? Help! Thanks!