[jeffcoslacker]

That is really odd. I don't remember ever running across anything like that, except of course for a cracked distributor cap letting moisture in, but usually that will just run bad or refuse to start at all in wet weather, not start fine and get worse.

I'm at a loss as to what could cause your symptoms. You seem to be describing something like what happens during vapor-lock, and I'm guessing that's why you tried loosening the cap. Did you notice any inflow of air as the cap was opened? Some Fords would do something like that when a tank vent solenoid stuck, developing vacuum in the gas tank until the pump finally couldn't overcome it, and the engine quit.

But leaving the cap loose or off would relieve that, so that doesn't sound right.

I'll keep thinking about it, and post if I think of anything.

Good luck with it.