75 Chevy 400 SB cooling issue

[Dan 75]

First post so hi everyone!
So here's my dilemma.
Just got a 75 c10 and I'm having a hell of a time figuring out a cooling issue.
Already flushed and replaced 180 thermostat, radiator cap, hose to overflow. It will build pressure, run at 160-190 at idle/ low rpm soon as I drive it and get it up the temp up to 200 it looses firmness on hoses and will suck the top hose shut and allow the truck temp to spike to 220-230 let off the gas the temp drops back to normal. Once it gets hot hoses collapse at 2000+rpm and won't build pressure. I'm at a loss if a bad water pump can cause this or bad hoses? It has a Weiand pump old school copper radiator. When I flushed the system it flowed fast both ways, but there was a lot of rust and crud in the system. Can a water pump intermittently pump fluid or could the pump just be in the beginning stages failure? Or is it as simple as the hoses have gone bad and don't collapse until they heat up?

Any help would be great,
Thanks