Power loss, valves ticking, feels heavy to run then stop with a boiling reserv tank..

Hi, I have some problem with my 1997 XJ Sport 4.0 recently. The car is running normal when I constantly drive around 80km/h with a temperature needle remain in the middle of the indicator. But if I hit the gas and let it run a bit aggresive around 100-120km/h for 15-20 minutes the temperature will get up one more bar more than half of the indicator then the car start to loosing power. The car feels heavy/hard to run and with a ticking sound from the valves, it's feels like a manual gear vehicle climbing a hill and need to shift down (my car is automatic gear). If I keep the car running on that condition for about 15 minutes the coolant in reservoir tank will get boiled with a gurgling sound came from under the radiator cap as well.. Have check the radiator water, check the radiator cap, check the engine oil everythings was fine, no leakage and not less than standard. My car also equiped with extra fan and oil cooler as I'm staying in middle east.

Can anyone advised me what happened and how to solve it? Thanks

If you're not losing coolant and the level inside the overflow reservoir isn't going down, it's just possible that your radiator cap isn't holding pressure and is allowing the coolant to boil. It's a simple cheap fix to replace it and see if it cures the problem.
Also remove the overflow reservoir and rinse it out thoroughly. Sediments can build up at the bottom and prevent coolant being sucked back into the radiator when the engine's cooling down.

Replace the radiator cap first, if it still does it replace the thermostat. Just did that on a buddies 96xj and solved the problem! I replaced both cap and tstat.

Seto, fill it up enough that the coolant level is in the fill neck where the cap goes. Start the engine and let it idle. If you continually get bubbles there, you have a bad head gasket. It sounds like when you are running it hard that the head and oil temps are getting too high, thinning the oil and you are either getting lifter noise or engine knocking from too high heat temps. The only cure for this is replacing the head gasket.

X 2 on rad cap and then thermostat.

Rad cap needs to be the type that allows flow, both ways.

Ensure you have enough coolant in the system ... as 4.3LXJ mentioned ... not just the overflow bottle ... and maybe check that the overflow tube isnt clogged.

Head gasket is possible ... That overheating issue you had a while back - may have caused some problems ??

Have opened the cap and let the engine run til hot, no bubbles.. Have checked the thermostat, it's working fine.. Will try to replace the cap tomorrow and if no luck then will replace the gasket and clean the water line.. Will keep you all updated..