ONLY a OEM jeep/mopar thermostat will work, I had the same exact trouble on mine when I first bought it.
I had a custom 3 row copper core radiator built,before I got an OE stat. The oe stat has a check ball for air bleeding, it alone dropped the temp 25 degrees.
At the xler dealership they call it the "heavy duty stat".

Let us know if that helps.

SD

Also as far as a head gasket problem,there is a kit to check for combustion gases in the coolent,quick and easy,any good garage should have one. <img src="/forums/images/graemlins/smile.gif" alt="" />

HAD THE SAME PROBLEM WITH MY 4.0 A FEW MONTHS AGO. YOU WILL WANT TO START THE JEEP AND LOOK UPWARD ON THE FRONT OF THE ENGINE(LAYING ON THE GROUND)TO SEE IF ANY WATER IS COMING DOWN THE FRONT ON THE OIL PAN. THIS WILL INDICATE THAT THE WATER PUMP IS LEAKING(AS YOU PROBABLY ALREADY KNOW IT IS HARD TO SEE THE PUMP). IT IS POSSIBLE THAT THE THERMO IS STUCK IF THE HOSE IS HOLDING PRESSURE AND NOT LETTING COOLANT THROUGH. THIS CAN HAPPEN EVEN IF IT IS NEW, I WENT THROUGH 2 THERMO'S WHEN FIXING MINE(DID HE USE LUBRICANT WHEN HE INSTALLED IT?). UNLESS A HOSE IS BADLY CLOGED IT SHOULDN'T BE THE RADIATOR. IF THE THERMO IS OPENING AND IT DOESN'T HAVE ANY WATER COMING FROM THE PUMP(PER THE PUMP WILL RUN AT ALL TIMES BECAUSE IT IS BELT DRIVEN)YOU WILL WANT TO PUT A GUAGE ON THE RADIATOR AND SEE IF IT IS LOOSING PRESURE. IF IT IS LOOSING PRESURE IT IS PROBABLY A HEAD PROBLEM(GASKET OR CRACKED). YOU WILL ALSO WANT TO DO A CHECK ON ALL OF YOUR FREEZE PLUGS JUST TO MAKE SURE BEFORE YOU GO TEARING INTO THE HEAD. HOPE THIS HELPS <img src="/forums/images/graemlins/smile.gif" alt="" />