The lowdown on cheap COPs

2003 Explorer 4.6L with 200K.
I recently changed out the COPs with an inexpensive set (about a tenth the price of Motorcraft COPs)

Now the motor runs fine for the first hour of driving, but then over the next hour, it steadily misses more and more with any increased load, like going up a hill at highway speed. The next day it runs fine again at first and then again starts missing after about an hour of driving. I believe the problem is related to the ignition, but it isn't necessarily the fault of the COPs.

So my question is: Has anyone tried these aftermarket lowpriced COP's that sell on various places like ebay and had this specific problem with them - that they start to fail after an hour or so of driving and then are fine at first the next day? Does it make any sense from a technical standpoint that a cheap COP would fail in this specific way?

Pardon my ignorance, but in this context, what's a COP?

Coil On Plug maybe..

Why did you change the coils? Was it doing this before the change or has it started since the change? Breaking down when heated is a common malady with some electronics.