[MidOHasp]

Things just sound too violent to be a matter of simply decreased cooling capacity.

When I had this problem I kept throwing "cooling enhancers" at it. Smaller waterpump pulley, less "coolant" and more distilled WATER, water wetter, lower pressure cap, higher pressure cap, lower t-stat, higher t-stat, etc...

All of these band-aids did very little to fix the hole in my engine. Luckily, torquing head bolts fixed it. If block is cracked or something, all of these little issues are moot.

Could be a stopped up radiator, but your inital post just seems to be pointing at a more serious issue. I hope I am wrong!

Good luck...