Originally Posted by T-bucket23
I would guess 1 of 4 issues listed in order of probability
1, Too little air flow through radiator or it is undersized. Possible
2, Internal cooling system pressure holding thermostat closed.
Running a 7-8 lb cap and it does not blow off through the overflow. If I watch the temp guage, as it goes over 180 to around 190, the temp then will drop back to 180 in about 30 seconds, so I think it is opening.
3, Radiator hose sucking shut at highway speed.
Brand new hoses for top/bottom/heater core.
4, Ignition timing retarded
This is where I'm going right now. Also, checking lean and vacuum stuff. Then to #1 with hood off trial at highway speed.