I've spent considerable time and effort to sort out our Delta TX. This includes fitting a remote digital wall thermostat to replace the defective thermostat in the unit. However, the main control unit is "sealed and non-serviceable", according to the manufacturers, and replacements are no longer available. I have read of sucess achieved by spraying WD40 through the joints in the unit, but I wouldn't bank on it. The control unit is the metal box at the front kerbside, visible when the internal plastic shroud is removed. With the number of faults in your unit, and the possibility that the external shroud is brittle and cracked from UV attack, I recommend replacing the entire unit, either with a new one, or one from a salvage yard.
Nick Crowhurst, Excella 25 1988, Dodge Ram 2500 Cummins Diesel. England in summer, USA in winter.
"The price of freedom is eternal maintenance."