You probably have 'air pockets' in the cooling system. Not uncommon after the work described. Cooling system needs to be properly bled after
its been evacuated. It takes about an hour to do it right... The cooling is closed loop, there can be pockets of air in this closed loop that move with the coolant and can cause the symptoms you mention. Air cools and warms much quicker than liquid, especially antifreeze. So the temp spikes due to that rapid change in coolant gauge fluxuation. That's the air pockets passing the coolant temp sensor and the sensor goes nuts cause the air is WAY HOTTER than the coolant he just measured. Then, the air pocket clears and real coolant comes into the picture, the sensor realizes that and the gauge goes down...then the air pocket comes around again and the...nevermind...