Who is telling you that the COP is bad? The Ford Tech? If that is the case, I question whether the COP has been bad each time. Are you getting a code and checking it yourself? If you're not getting a code and it's missing right now, I question that its the COP as it usually spits a code rather quickly, or at least it does on my 2000 E-150 5.4.

Also, did they replace the boot between the COP and the plug? I guess there's been problems there too.

Are they doing this under warranty too? I know the ones I buy at Advanced are 1 year warranty. Of course I replace them myself.

I can't think what would cause a COP to go out so often, especially after replacing the connector and plug. I also don't believe the connector was bad the last time. If it was melted or corroded, then maybe.

If I were you, I'd swap COP 5 and 6 and see what happens, but that's only if your getting the P0305 code or P0355. Then see if it changes to P0306 or P0356.

I had a strange one recently where my water froze and the engine overheated and went into limp mode. Once it was back to normal, it was missing and coded a P0308 and it had blown the #8 COP. Not sure why that happened but a new COP fixed it right up.