Got to disagree that "if the gauge is working at all, its fine."
The gauge could be working fine but the capilary tube may be shot and I don't think they can be replaced separately.
My gauge was cycling also. Flushed the system, cleaned and flow tested the radiator, changed the thermostat, etc., all the basics.
Before changing the head gasket and as last resort, I replaced the gauge with a new one and everything was normal again.
Turned out that the cooling system was operating properly all the time.
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'86 745T, '72 144E, '70 145S, '68 122S Estate, '60 544
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