Your last problem could be the TFI module. Your first problem could be in the wires, too. I agree that the heads are the likely problem, but if wires #5 & #6 are touching at all, a missfire/crossfire situation can occur. The better the wire quality, the higher the rpms will be before a problem is detected. Those two cylinders are sequential in the firing order, and are next to each other in the engine, which is one of Fords few designing flaws. Honestly, that problem tends to present itself more as a pinging, or detonation problem, than a backfire problem, but it's still worth noting.
Gppd luck!
Take care,
-Chris
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