overheat only over 3000 rpm

Hi
thanks for the reply.
No, it wasn't sucking shut.
A long time ago I hadn't the spare to put on and I adapted (on the road) a Ford Transit coolant hose, which has he same bends. Only a little longer.
AS it came out, when is new and helastic it holds the rear bend without problem. Then, after some time ( heating/cooling cycles) it collapses in that bend and reduces the flow.
At least, that's what happened to me....

For me, it was an old funeral car, early 50s Cadillac Hearse.
It ran fine during the procession out to the Burial Park,the other cars could return on the hiway, but this one overheated at speed.
When brought to us, we rifled thru the glove box, found the newish looking rad was just a coupla years old by the receipts etc.
Test drive had the fan belt squealing like hell, it was loose, adjusted it, fat belts.
I was watching the generator pulley as it seemed bent at speed.
It was actually the lower hose sucking shut directly below the pulley that my eye was seeing, but my focus wasn't on the hose, whoda thunk hose.
After watching a while, I could hold the revs at about 2000, and after a few seconds, the hose would collapse, - then open as soon as the revs dropped.
Happened a few years later on a 6 cyl too.
Showed the crew .