my 86 z28 does this strange temp flucuation while driving down the road. at random while driving temp gauge goes from normal of 160ish then comes up to over 260 stays there for about 40-50 seconds then goes all the way back down to 160. doesnt happen all the time and after it happens once it could be 20 min or so before it happens again.
cooling system specs. 3" griffen rad, weiand hi-flow aluminum water pump, milodon hi-flow 160 tstat,stock electric fan.coolant sensor in drivers cylinder head

 

BEFORE you do ANYTHING ELSE...

Install a mechanical or electric ( I like mechanical ) AFTERMARKET temp gauge, even temporarily to see if the gauge is just acting foolish as MOST GM gauges from the 80's do.

 

Just food for thought, but it is possible to have a faulty thermostat also, even if it is new!!! Happened to me. Also I second that thought of hooking up another gauge just to see.

 

the thermostat i took out looks to be the exact same as what i put back in. it has the same numbers stamped on it. engine did the same thing with the old one.

all of the parts listed were replaced mostly one at a time as they needed replacement, not to try and fix this problem.

thanks for the pointers i'll try a different gauge once i put my car back togather(currently tore down to get full paint job).