Heating and Engine Operating Temp Issues

[horse with no name]

Hi All,

I have a question that likely has been asked before, however, I couldn't find any threads that detail my exact issue. I have an '88 5.0 with minor mods (Exhaust, Gears, Short-throw shifter, Fluidyne Aluminum Radiator).

Recently I have began to notice the temperature fluctuating between 160° and 200° after the car has warmed up. The engine temp even occasionally dips back down to about 140° after approaching 200°. In addtion, the heater takes forever to work(about 20-30 minutes), and even when it does, it occasionally begins to blow cold while the engine temperature continues to fluctuate.

Initially I was thinking this was maybe a thermostat issue. However, I am concerned the heater core maybe beginning to go out (no signs of leakage though). Haven't changed the thermostat yet.

Any thoughts?

Thanks in advance for your help!