suddenly loss of power

I have a 96 Isuzu Hombre truck with 91,000 miles. It has been running surprising and pleasantly great, but today coming back from a job it suddenly has little power.
Other than that it sounds fine except for a small tapping sound which it has had for several years. There are no loose cables or tubings, no funny smells, nothing usual. I could barely get it up to speed on the freeway. It will still do 70 miles an hour once I go through all the gears.
Any small hill, I have to down shift to make it up.
Any ideas what could have happened...feels like it is not getting gas. could it be a plugged filter??
Thanks
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I assume it's not missing on one cylinder? It would be obviously vibrating badly, etc if that were the case. My wife's Trooper lost spark in a cylinder...it was all I could do to get it up to 65 or so in 3rd gear (4-speed auto). Ended up one of the coil packs was bad and the plug wires were in need of replacement as well.
Wesley

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Have you checked for a vacuum leak.

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