2005 Mercury Mountaineer Power Loss

Hoe long since it had a tune-up? Worn spark plugs are the best suspect.

The next thing is to have the diagnostic fault codes read and recorded. Those do not tell you parts are bad or need to be replaced. They indicate the circuit or system that needs further diagnosis. They can also tell you which cylinders are misfiring.

I'm not understanding the chain of events. You're saying there was a misfire, there was a diagnostic fault code referring to cylinder 8 misfiring, you replaced the spark plug and wire, the problem was gone, then this new problem of failing to accelerate along with another misfire has occurred. Is that right? If so, you have to start over with the same diagnostic steps. Logically you would want to replace all of the spark plugs and wires. Next is to erase the previous fault codes if they weren't already, then see if new codes set.

If the previous codes weren't erased, the simple fact that they're in memory can cause the Engine Computer to suspend some self tests that rely, for comparison, on anything it thinks it can't trust because of those codes. Once the old codes are erased, the rest of the tests resume, and that's when new problems will be detected.

Also, your engine uses the scary "throttle-by-wire" system that put Toyota in the news a few years ago. If that system shuts down, the throttle won't respond when you press the accelerator. That shutdown can occur because of a problem or as the result of some other problem. Either way, that is something you want a professional to look at.