I dont' know. Something about it is bothering me. If you can get spark and fuel to all four cylinders so it will crank over and fire with ether or whatever, and it'll run up to operating temperature. So you've got fuel, spark, enough timing to keep both somewhat in line, but cold and hot you've got a problem.
So, it has to be oil, fuel, or electrical. A sensor would have to be very odd to have faults at two points within the range (as I'm sure you know), but mechanically, you might have variances.
Is the VCT working properly? The reason I ask is that there was a TSB on it way back when. If the oil pressure is varying and it is slipping the cam lobe off the tappit, or isn't aligned well enough to open and close properly, it could be a problem. Also, oil chambers in Ford's of this vintage can have really finicky oil passages, so you could have a clog. But that seems unlikely given the mid temp ability, but not the high temp.
Definitely confusing as all hell.
Let me know your progress and I will continue to think on it.
As for my own predicament, there was also an old TSB that dealt with stuff I thought was just the character of the car. I'm going to try a reflash and hope it works.