390" running hot

Hiya!
Need some suggestions on where to start searching for faults.

Here's the problem:
I have a 1960 Cadillac CdV 390 engine original, as long as I'm driving slow and just idling the temp is fine (normal) but when I drive faster around 55-60 the temp goes up all the way to the top, so I pull over and the temp immediatly goes down to normal again when idling.
Almost seems like the thermostate wont open to high rpm, anyone had this problem before?

Thankful for any help.

You may have answered your own question.
There is insufficient coolant flow somewhere.
Change the thermostat as they are cheap, and try driving it.
Is your radiator getting old and might be blocked? If so, remove it and have it boiled out. Maybe flush the block at the same time and clean out any rust.
Also make sure the waterpump belts are tight.

Shyte! I found the problem, after spending a lot of hours cleaning the radiator and flushing the whole cooling system.
The problem was simply the charging regulator, giving the wrong current on higher rpm. Causing the temp gauge to show the wrong temp.

I should have thought about it, but I've never seen that happen before.
Thankfully these old regulators are easily adjusted.