I would check the filter and purge any air from the unit and see if that restores normal operation. Is your sprayer showing any signs of being clogged?
no sign of clogging, air is already purged and pressure good, temp / pressure good for the first couple of mins and then goes haywire
no sign of clogging, air already purged, pressure/temp good for the fist couple of mins and then goes haywire
What happens if you set the temperature controls to maximum. Do you at least get consistent flow and temperature?
The only reason I can see such a wide variation after a couple minutes would be due to a mechanical temperature switch not responding like it did when the unit was new. If you turn up the controls to maximum the unit should give you consistent flow and temperature. Then I'd try the opposite....see if the cool setting is consistent. That would at least verify the heater and pump aren't the cause of the issue and isolate the mechanical thermoswitch.