Chasing this cooling issue for months

Or maybe should call it a "heating issue" I don't know anymore.
318, 8 blade impeller, new 26" radiator, new 17" S blade 2400 CFM electric fan set up as a puller, 195 thermostat.
Also mechanical temp guage, not electric.

I can drive around town at low speeds in stop and go traffic and the temp stays right where it should, 195-210.
It will slowly come up to 210 then the fan comes on and brings it back down to 195 in about 1 block. (Seems great so far)
BUT, if I get out on the highway at 55-75, within 1 mile the temp is down to 175.
It can be 90 outside and it drops the engine temp to 175 at highway speeds.
Also I have waterflow through the radiator at 175 degrees and that makes sence as to the symtom.

Question is why?
With a replacement thermostat because I thought the first new one might be bad, it still overcools.
How is it possible that 175 degree water is getting past a 195 thermostat?
I know for a fact that this is what's happening due to the fact that I have a tiny flat spot just off the throttle stop at anything under 190 degrees and when the guage starts dropping under 190 the flat spot is there.

I'll bow (In Public) to anyone that can school me on why.:prayer::D

Thanks