1989 ford f-150 start troubles

this is a problem that has plagued this truck for about 2 years now. I have
posted this problem on about every ford tech site on the net. Still no go.
heres the situation,
the truck will crank right up, and drive perfectly to wherever I am going. I
then turn it off.
If it has been about 30 miles or so, it will NOT restart until it has sat
for about 15 minutes(as if it needs to cool down).
it tries to turn over, but will crank like a low battery. Jumping it does
nothing. It has to sit for about 15 minutes minimum, Then it will crank
right up like nothing was wrong.
Of course this doesn't happen if its a short trip. Only if the truck has
been driven 30 miles or so.

now heres what I have done/changed:
the obvious guess would be the starter, perhaps bushings heating up..but,
I've been through 2 of them..NO DIFFERENCE! one remanufactured, the other a
new motorcraft.

next, is the battery..brand new, new cables and leads. ground is good and
tight to motor.
I even ran a bigger gauge lead to the starter, no difference.
I've changed plugs, wires, solenoid under hood, and still NO GO.
Someone suggested that it had jumped time...checked it with light and its
perfect.
distributor you say? NOPE..new one too.

any ideas? it sucks to HAVE to stay somewhere for at least 20 minutes.








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