'89 Trooper - Fluctuating power..

My project is an '89 Isuzu Trooper (2.6L 4cyl. manual) with 104,000 miles. Had to replace the thermostat a couple weeks back, and it's been running well - until recently. It runs strong while cold, but sometime after it
warms up the power seems to fluctuate, while idle the rpm's will bounce between 5 and 15, and when running I feel the same "bounce" in the power. I tried a bottle of gumout in the gas tank thinking that the problem is with the injectors, no luck.
Any ideas?
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