Erratic coolant temp

[bmeyer29]

My coolant temperature is very erratic sometimes. Generally it runs around 80C after the engine is warmed up, but occasionally it will bounce up to 90C (in about 5 seconds) then drop back to around 80C (again in about 5 seconds). I haven't worried about this and assumed that was just the normal opening and closing of the thermostat (180F). It did the same two days ago at highway speeds (over a 25-30 minute drive), but today on the same route, it it went up to 90, then down a little, then up to 95 and down a little, and up and down... It got as high as 110C. I had been cruising on the highway for 20 minutes or so, and was going around 70mph in 5th gear at around 2000 rpm. I pulled off the road and let it cool for a while, and then headed home. Going home it was back to staying at about 80C, and then every few minutes it would bounce quickly up to 90C then back down to 80C ( over a total of about 10 seconds).

Is this a sign of a bad thermostat?

Details:
351W rebuilt locally, dynoed on the stand at about 450HP, about 350miles on the engine since the rebuild, 750 Holley carb, large aluminum radiator, ducted fan (and fan on!), 180F thermostat with small check valve at 12 o'clock, 50/50 mix, coolant within 1/2" of radiator cap when cool, 16lb radiator cap, metal pipe for most of the lower radiator hose so unlikely to have collapsed from heat/suction.

My gages are Autometer and in Celsius, so I have to translate them to more familiar numbers:
80C=176F
90C=194F
100C=212F
110C=230F

Many thanks for any suggestions. I am always impressed with the willingness of forum members to help out others!