overheating mystery

:evil: I REPLACED THE FUEL PUMP AND THE FUEL GAUGE JUST ZEROS OUT NOW BUT IF I DRIVE A WHILE IT WILL WORK UNLESS I STOP THEN IT WILL ZERO OUT AGAIN THEN IF I DRIVE IT WILL READ IT AND IT HAS BEEN RUNNING MUCH HOTTER THEN BEFORE I REPLACED THE FUEL PUMP NEARLY OVER HEATING

Hello, all: I have a 2001 Grand Cherokee, 4.0L six with air. The other day I passed a car and shortly afterwards, the temp gauge pinned at 260. I took it to a "pro" and he did a block and pressure test--they both came out good. He suggested a new radiator--got one and put it in--no change. I...