Idle and Horsepower Problem

I own a 98' Explorer V8. I have had a problem with what I though was "missing". So i changed out all the spark plugs and this worked good for a while. Now however, it seems to have come back with a low idle and also it sometimes dies now. I also seem to have lost some horsepower, visible by the pick up, which I am sure has to do with this problem.

I am not sure what the problem is and rather than replacing parts at random like the fuel pump, Idle Air Control Valve, spark plug cables ect. I would like to know what is causing this. Is there any way to test these to help eliminate possible causes? Any help would be appreciated.

Thanks

Have you figured out your problem yet. I have 98 Ranger doing sort of the same thing.

Is there a Check Engine light on? If so, get a scan and post the codes here. If not, then it may be fuel system related (Fuel Filter?)