One of the things I always consider, when someone complains of 'loss of power' on an older rig, is a clogged catalytic convertor. Typically, older rigs will have only one O2 sensor, thereby allowing a clogged cat to NOT trip any codes. Simply unbolt the exhaust at some point between the engine and the cat, and run it with the exhaust open. Yeah, it'll be noisy. But a 'short' drive won't hurt anything. If power is restored, then you've found your problem. If it still runs crappy, button everything up, and you've eliminated one potential problem.
Another thing to consider, is carbon deposits accumulating within the engine, causing a valve to stick open. Easiest cure, is to use some top engine cleaner. A 'burnt' valve is pretty rare, with today's computer contolled engine. However, running shitty fuel can easily cause carbon deposits. Typically, running good fuel (Shell, Chevron) eliminates this issue.
1992 YJ, 401 V8, EFI, TH400, Atlas, D60s, Detroits, Warn 8274, Cage, 37 Krawlers
1997 TJ, 350 V8, EFI, TH400, Atlas 4spd, D60s, Detroits, Warn 12000, Cage, 40 Kevlars
TTC 1994, TTC 2012, Trail Tour participant.