I have a 93 delta 88. heat comes and goes. we have changed the thermostat, and checked fluids. the car does not run hot. but the heat will work one day and blow cold the next.

Check the heater control valve in line with the heater
hoses. With the heat on, the hoses on both sides of
the valve should be hot. If not. check the actuator on
valve, if it feels loose remove the valve and check the
shutoff flap inside to see if it is loose on the shaft.
This can cause intermitent heat. For some reason I
can't find the diagram for your car so I'm working
from memory. But I hope it helps.

Check the vacuum diaphragm in the heater hose. It's a little metal valve with a hose going to it. When the heat is turned on the vacuum line opens the valve letting hot water pass through the heater core.