1) I have had a problem with a mustang years ago,and it was the electronic
Iginition module on distribuitor.
2)A bad coil can cause the same problem!
When heat go's up in a coil,the inductance reactance go's down.
The voltage might not step up thus cause a misfire.
Note: Every time the engine would get Hot,I would lose the Timing!
GOOD LUCK!
MCGIVER