Dark_Knight_666

Hey guys, well I have a bagged 84 2wd pickup with a 21ru. I just installed the LCE block off plate kit, a Weber carb and aftermarket gauges. I replaced every gasket on the intake side and water pump gasket aswell as a new 180° thermostat. When I installed the temp gauge I put it in the port on the intake on the top as far forward and closest to the thermostat. The factory temp sensor is on the side next to the thermostat. It still works fine and new fluctuates but the second mechanical temp gauge is fluctuating as I drive it. It'll climb to 210° then drop to 180° then repeat the whole time except stopping at 200° then dropping to 180° once it's completely warmed up. The stock gauge stays planted between 1/4 & 1/2 while the other gauge is doing this. I have no air bubbles in the system and no leaks, coolant is totally full. Any ideas? Should I try the port right behind its current location or on the heater core port directly in the middle of the intake manifold? I'm at a loss. Thank you.