Gutless low power Jeep Cherokee Sport with fuel injected straight 6 what is up with that?

What I want to know is what is the possible cause of why my Jeep Cherokee Sport ’97 lacks power and umph when it gets to regular operating temperature? I will try to be brief with the details but here is the particulars.

My next door neighbor had the Jeep, he use to drive it to work about 26 miles away. It started to over heat, the tappets would get noisy and then the thing would putter to a stop. He was always adding fluid so he thought he blew a head. He offered it to me for a mere $150 I wish it were the heads because it would be a straight forward problem. It turned out to be more sticking thermostat and failing water pump was the problem. So, I replaced them, over heating problem solved. However, when it was overheating it had power, I could go ripping down the freeway and keep up just not very far because the Jeep would overheat. Now that it doesn’t overheat it runs too rich I know, I seen the signs when I replaced the plugs, cap and rotor. Since the lack of power was not the cap, rotor, and plugs which were indeed bad and replaced or a bad fuel filter I am thinking it maybe the fuel pressure regulator because it is running too rich as oppose to starving of fuel. With your mechanic brain what do you think? Vehicles with a 6 cyl. (straight), fuel injection that loses power can most likely be what? And I have put better octane fuel in it, and injector cleaner and performance still has not improved after the engine warms up, same old sputtering and stalling. What do you got, I am kinda stumped?