Coolant temperature fluctuations

[66gmcguy]

Hey all. I have an 82 K2500 with a newer vortec 350 from Blueprint engines (around 4000 miles on it by now), and just today started getting massive temperature fluctuations. It only happens when I’m cruising above 50mph or so. It will swing from 200-240 then back down to 190ish in a matter of seconds, and it will do it the whole time I’m cruising. It doesn’t do it if I’m just driving around town or idling. The sending unit is in the drivers side head. Now here’s where I’m stumped. I have a holley efi system and it shows you the coolant temperature on the display. Whenever the temperature swings hot on the gauge, the EFI display simultaneous shows a temperature DROP of only about 15 degrees (195-180), and once the gauge (in the cluster) hits about 240, it will start to drop again, while the temperature on the EFI display climbs back up to 195. The sending unit for the efi system is in the intake just below the thermostat.

Everything on this engine is new. I have a griffin radiator with electric fans, high flow water pump, new 195 high flow thermostat (with bypass), edelbrock aluminum intake, and I’m using the heater core (which is also brand new) as a coolant bypass. Is it possible that the thermostat is sticky (but not stuck) since the temperature is fluctuating around the opening/closing point of the thermostat? The coolant level is completely full.
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