I had some major problems with my 408W that sound somewhat similar to yours. Turned out my head bolts were not tight. 20-40 ft-lbs on some of them. Bad news. My water temp gauge in the car would get up to 120C regularly and actually pegged at 140 once. That's very bad. Once that happened, I started checking everything.
Just happened to see some oily "paste" under the valve covers, so tore them off and checked head torques. They were barely even on. Torqued everything up to spec and have never had the problem since.
When I was seeing 120C on the temp gauge, the radiator was ice cold. So I was thinking flow issues. I was right, but had no idea WHY I was having them. Thought bad pump, cavitation, lower rad hose collapsing, everything. Slowly eliminated every possible variable and it was STILL happening. Only by chance (
oil change) did I notice the
oil paste. First thought was blown head gasket, but turns out there was just not enough torque to keep the power in. I was blowing straight combustion/exhaust gasses into the cooling system. No wonder the manifold temp was reading so high and so quickly (immediate 50 degree spike if I breathed on the gas), the radiator was cold, and I was losing a gallon of coolant through the cap when I went out.
I put a 22 pound cap on, still lost coolant. Put a 26 pound cap on and blew the surge tank clean off the gooseneck (held on with a piece of hose and 2 clamps). I thought there was a blockage causing the lack of flow and insane pressure. I was just blowing smoke in there the whole time.
May be worth checking into... Our engine ran fine on the dyno, 13 pulls. But once it cooled off, it needed retorqued. The guy who dyno'd it said I'd be fine without. He was wrong. Dead wrong.
Check your head bolt torques. Could be it...? Maybe not.
JP