Temp Gauge Goes Cold when Accelerating

Not sure if this has been covered before but I changed out the radiator in my car with an aftermarket one when my old one burst. I replaced the thermostat as well now when out to make a test pass my when I accelerate my temperature drops down to about 140 amd returns to normal once I let off. Anyone have this problem before or know whats causing it

Temp Gauge Goes Cold when Accelerating