This is scary as my 97 XJ started doing the exact same thing about a week ago when I was climbing a gulch out here in the desert, it wasn't hot out and the climb was difficult and my rpm wasn't high either. I thought I had punchered the oil pan so I got out and checked for oil on the ground and checked the oil level but everything was fine. I'm thinking of changing my sending unit and also adding a mechanical gauge. Maybe someone else has had this problem and knows the fix.