’97 Jeep Grand Cherokee 4.0 V-6 191K miles. Overheated. Filled it with coolant and it overheated again the next day. Saw a green puddle under the vehicle but could not find a leak. I added “Bars Leak” just to get me through to the weekend. Vehicle would run about 12 miles then overheat again. Saturday I put it on ramps and searched for a leak. Found a pencil-point-sized leak on the back side of the engine just below the valve cover, in the center of a freeze plug. Filled the hole with JB Weld, flushed the system, refilled the coolant, and no more overheating. But as soon as I rolled it off the ramps another problem began. The Jeep starts fine, idles fine, and accelerates fine until I drive it for about 3 minutes. Then the rpm’s drop to below 300, I step on the gas and nothing happens at first then the car lunges forward. It almost feels like it’s about out of gas and inching its way to the station. If I floor it, it lunges forward then back off. If I pump the gas peddle the surging stops but I get nowhere fast. I pull off the road and let it idle and the engine runs rough and the rpm’s drop again. This continues for several minutes then the engine revs up by itself and then idles just fine. I start to drive with no problems for about 3 minutes and everything starts over again. I replace the O2 sensor aft of the Cat Converter, no change. Checked the tranny fluid, changed the oil. It seems like a sensor problem somewhere. Would the O2 sensor before the Cat Converter cause this? When I change one should I change both? Maybe a short to the fuel pump? Could the overheating problem have initiated this power surge problem? Any ideas?