what is best to do when you have motor problems out of the dodge durango. on 2003 Dodge Durango

I have already changed the head gasgets, had the heads serviced. Put a new thermostat in. Put in a water pump and put a new water hose on and my truck is still not running well. Confused what should I do

rec. fuel system clean. ethanol treatment to 1 tank premium fuel. ethanol blend in fuel causes build up in fuel system, combustion chambers, exhaust system and causes increased wear to spark plugs. clean fuel system, better burn.
rec. oil stabilizer in with next oil change. this will recondition seals and cylinders. improve compression.
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check for codes most auto parts stores let you use there obd reader