B20 Help Please

1973 D-Jet
I can get the car to run and drive great for about a mile and then starts running like it's on 3 cylinders or flooding out, no power. I've noticed when it starts running bad, the temp gauge drops considerably. I can let it set for a few minutes, start it, gauge will be in the middle of the scale, it will run fine and then same thing again. The temp drops and it's over. The car ran and drove great for a week or two then this problem showed up.
I have replaced
-plugs, cap , rotor, pertronix points, wires
-injectors and seals
-temp gauge sensor and engine temp sensor
-fuel pressure regulator (pressure is good)
-thermostat
-all resistance and voltage checks check out from ECU to sensors
-fuel return is not clogged
-EGR is removed and capped off
-electric air valve in place of aav
-MPC has been modified to be adjustable

I would greatly appreciate some help before i burn megasquirt money.

Have you checked the fuel pressure while this is happening?

Did you check the fuel filter? If you haven't, remove the fuel filter and let it drain backwards through the inlet side into a clear bottle. If there is a lot of debris, the gas tank may have a problem.

Did you try a vacuum test on the VPS(Vacuum Pressure Sensor)? If it doesn't hold vacuum, then it has a broken diaphragm.

The temp gauge dropping may indicate a voltage supply problem. Put a volt gauge on the coil + terminal and see if there's a correlation between voltage and the way it runs.

The battery junction block is a common failure point. Make sure it isn't grounding against the inner fender.

I was going to say - I don't think there's any way the engine's actual temperature could be dropping like that.

looked at the plugs to actually know if it is rich or lean?

Had the same problem on my 245 - B21A, alternator + wire was damaged. So wiring is the key point in my opinion.

I appreciate the help. Plugs look good. I've swapped them then out just to make sure. I also have replaced coil. The temp guage issue is what's throwing me off, leads me to believe it's electrical. The fuel return is clean but I haven't checked the filter yet.