The car is a 55 Bel_Air running 96 model yr small block, vortec heads, etc ...I have heavy duty radiator mounted in 6 cyl position. It is a down-flow radiator. I have a Milodon 180 high flow thermostat. I have a Sunpro mechanical gauge setup ( Water, Volts, Oil ) under dash ... looks nice ... Volts and Oil appear to work ok ... The weird thing is the temp gauge. Originally, I placed it in the intake manifold. It would fluctuate from 160 to 250, usually settling around 190ish, but periodically climbing north from 210ish to close to 250 ... It spikes when cruising ... 55 mph or higher ... Then it would quickly ( and I do mean quickly ) drop back, sometimes as low as 160 ... then climb back to near 190 ... So, I moved the sensor from the intake manifold to the head, thinking maybe I was catching air pockets. Now, it is not fluctuating up and down as quickly as before, but the basic symptoms remain the same. The gauge is slow to initially read, but, after a little cruising, the temperature swings began. I have an SMI Stage 2 carb and I am running as much timing as I can get away with. The vacuum advance seems to be working properly. I have been debating whether the thermostat cycles slowing ( grasping at straws ) or insufficient coolant flow ( I have tried 2 NAPA water pumps ... )... The gauges are new ... water pump ... carb are new ... the radiator was professionally cleaned .... I do not see any evidence of coolant boiling ... I guess I'll replace the gauges, but if any one has a suggestion that I should look into, I am definitely open to suggestions ... As always, thanks in advance .... Steven

Steven, Are you running a heater core?

Those late blocks don't have the bypass drilled in the front of the block to the water pump like the older blocks do.

A bypass from the intake manifold and return to the water pump is needed.

A heater system with a normally open control valve (or none at all) should take care of it.

I've been there, done that.