Radiator damaged 3 times

Hi Guys,
I own a XJ classik year 2000 4.0 lt. I have relpaced the radiator 3 times (the first two I had to pay a lot at the official Service... the third was guarantee). As the 2nd radiator only lasted 3 month, I asked the Service to check what causes the problem, but they seem to replace the Radiator as a non-thinking beeing. Again, 1 week after the last radiator is instaled, I noticed a lack of coolant from the lower gauge. Something is causing an "over pressure" in the system (earlier I had to replace the cup). I dont want to wait until breaking another radiator. What should I check or replace at "my all time garage"?
Thanks
PD Temp meter normal, never overheated

If you have a leaky head gasket, exhaust gasses can get into the antifreeze. This will cause it to get acidic and eat the metal in the radiator. This can be tested using a Carbon Monoxide tester normally used to test exhaust emissions. You pull the radiator cap and place the probe next to the coolant. If the tester reads any hydrocarbons, your head gasket is shot.

Also what kind of water are they using ? MIght want to pick up some DISTILLED water from the food store, use that instead of tap water. But for sure check the head, they had some issues with cracking, take a flashlight and look straight down the hole you put oil in, look for any green.