Land Rover Repair: Land Rover Disco Overheating, radiator fluid, head gasket

Question
Have a rover missing some radiator fluid.  Read the blog on engine failure.  Try to find the leak by running the car in the garage for a period of time in drive at 3200 rpm for over 20 minutes.  No leaks and no probems.  When I actually drivce it though, some fluid will come out of the overflow hose on the resevoir tank.  Already replaced cap and thermostat.  Another expert mentioned could be early signs of engine failure.  Why does it not lose fluid when in drive if this is the case?  Any other way to check this?

Thanks
BOb

Answer
It loses fluid when you drive, but not at idle because the engine is under load.  Check for cold spots in the radiator once the engine is at running temperature.  If you find any, replace the radiator.  You can also perform a block test using test fluid which can be purchased at any auto parts store.  If it changes color, you have head gasket or engine failure.  Also, you can make sure you bled the system correctly when you installed the thermostat.