[Rick Parker]

I had the exact symtoms: fast rising temps after thermostat opened, temp gauge would swing wildly and peaked at 230-240 within a few seconds and then the gauge S/W mechanical would swing down to 140 and start the process over. Mine was on a fresh build. 289Hipo with the aluminum waterpump without a backing plate (1st generation). I had not run the combination previously. I fought it for several months, but the solution turned out to be a different waterpump with cast iron impellor instead of "sheet metal type", the one being used was a rebuilt unit from an Auto Parts store. The pump was apparently cavitating. I ended up purchasing a OEM Ford pump. No problem since then.
I will assume yours has run OK without prior issues(?) until this point. Check to be sure your pump is still functioning (impellor tight on shaft). Check your head gaskets.