Sadly, it's unlikely to be something simple as a fuse. It's extremely unlikely a fuse would partially fail...they either work or they don't. Why do you think it might be a fuse?
If it has the 4.0 OHV (pushrod) V-6, a common failure were the rocker arms and pushrods.
Less expensive problems could be a clogged fuel filter, old spark plug wires or old spark plugs. It could also be clogged fuel injectors or a number of other things. (This is kind of like a doctor guessing what's wrong with an 85-year-old man. The answer may be "all of the above...and then some".)
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