My jeep randomly overheats. It typically happens when I'm cruising around town or I'm idling. The temperature gauge red-lines for a short period then returns to normal. I've replaced the thermostat twice. I've also replaced the fan clutch and the waterpump. The engine doesn't seem to be overheating, but sometimes after a long drive, (for like 2 hours) I'll stop to get gas or something and when I start it back up, it acts really sluggish and sputters. Please help.