Odd temperature activities
I have noticed a similar behavior with my 67 dart 273.
All stock 2bbl, HP stock copper /brass radiator.
What I notice is on the interstate at about 70mph, the temp will rise above "its" normal place about a needle width above low normal (67 guage has an offset high normal that was corrected in later gauges)
It will get to about 3 to 4 needles width above its normal. Then it will drop with no change in driving or road conditions.
Caveat...
I toasted the temp gauge about a year ago and it sits 1/4 inch below the normal key off position. But it still sweeps and I know where its normal is.
So in my case it could be a bad gauge.
But I got to thinking, what if the lower hose was collapsing then opening back up?
The water pump is unknown age/miles, the rad was rodded out in the last 2 years, thermostat is 4 years old.
I never noticed this behavior before BUT I also never had a reason to look, now with toasted gauge I'm a bit over cautious and am checking it often.
I did do some driveway testing.
At idle I had a 44deg inlet to outlet Delta, the temp rose to 215 and stayed there for 30 minutes on a 90deg day. And the gauge was at the line I now call high but not scary.
I have a GoPro camera and am thinking of putting it under the car to watch the hose as I drive.