Stranger things have occurred. I am not a Ford guy, so I can only speak in generalities....
I had a very similar problem with a stock Chevy 305. Hot days it ran cool. Cool days it ran hot. Rev the engine and it would cool off. Temperature would spike and then drop down. Spike, and then drop. Only on cool days, never in hot weather. Supposition: sticking thermostat.
I had a row of 'stuck' thermostats lined up on the shelf. I then went after everything else - radiator, hoses, etc. I tried drilling 1/8" holes in the thermostats to allow for pressure equalization. No luck.
Since everything else had already been replaced, I swapped out the water pump. I put on a new 'Flow Kooler' brand high performance water pump. Problem solved!
My point... When all other possibilities have been eliminated, try something different...and use it as an opportunity to 'upgrade' parts!
HTH,
-Eric