Over Heating '89 Trooper...

'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.
Oil pressure appears fine, oil level is good.
Any ideas?
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