I have a 1998 4.0 SOHC that's been getting low on coolant every week or so. I've never seen any water under the vehicle so today I finally ran it around the block and immediately checked under the hood when I got home. I found the coolant reservoir completely full and water seemed to be leaking out from the reservoir cap (the outside of the tank was wet and dripping). The water was bubbling like it was boiling but it was just air bubbles (the coolant was cold). I could hear a hissing that sounded like it was coming from the radiator cap, but I couldn't pin it down...when I slowly relieved the pressure by turning the radiator cap, the hissing stopped, but the reservoir remained completely full. Then I noticed the hose going to the passenger side valve cover had oil around it on the valve cover. I pulled the hose and it had very watery oil in it, However my oil isn't milky at all at the dipstick, and no white exhaust. I have noticed a few times in the morning when I get a few miles down the road the red "Check Gauge" light comes on briefly and the temp gauge is pegged to HOT, then immediately drops to normal...clearly the engine isn't overheating so I don't know what is going on. There are no oil leaks that I can see and no oil is being used excessively. I recently replaced my thermostat and when I did that I completely flushed my water heater (tested it to make sure it wasn't plugged too) and radiator and the entire coolant system. 1) Could a bad radiator cap cause the water reservoir to overflow and the temp gauge problem? 2) Does the hose that goes to the passenger valve cover normally have oil in it, and should that be clamped onto the valve cover (mine is just pushed on, no clamp)? 3) What should I do first?