Geothermal heater losing temp over time

[Balakay]

I have an open loop Bard GSVS421-A geothermal unit that is about 16 years old and acting up while heating. I wonder if anyone has seen symptoms like this and has suggestions. With an entering air temp of 65°, the air leaving temp is at 90° (Delta T of 25°) for about an hour. Then the LAT starts to drop from 90° to 85, 80, 75, 70, then shuts the compressor down. Water temp does something similar, starting with a high delta T and reducing to no delta T. This happens over 2 to 3 hours while the thermostat is still calling for heat. Then to reset it, I have to shut the power off for an hour or so. I had a technician come and they didn't find any codes. I have 6gpm water flow, which is recommended. Water flow is still good when it starts losing temp, so no frozen lines. Airflow feels OK. Any thoughts on what might cause these symptoms?

 

[Fitter30]

Ever take enter and leaving water temp on the condenser? Has the water side ever been cleaned with acid? When startup was done was there a performance report done? And do u have it? Service person put gauges on the system? Take any temp readings with a Probe style thermometer on refrigeration lines or amperage of the compressor? When power is cycled off all failures are gone from what ur describing unit might be short of refrigerant starting to freeze the coil water side and trip off.