1998 Jaguar XJ8XL Low Coolant Level

My son's 1998 Jaguar XJ8 XL (48,000 miles) loses coolant regularly. I am adding coolant about every two weeks. Not a lot, but it drops enough to set off the "check coolant" message. No noticeable leaks or wet patches under the car. No white smoke at all, but there is a small amount of brownish gooky stuff on the inside of the radiator cap - very small. I understand this could be a variety of situations; hose leak, radiator leak, fuel pump issue or the worst scenario which is an intake manifold or head gasket issue. With such low mileage I am curious if others have had the same issue, and if it is an intake manifold or head gasket issue how I can make sure it gets properly diagnosed so I don't go through one costly repair only to find it didn't correct the problem. I have access to "import" mechanics in my area, but no Jaguar dealers. I am also curious as to opinions on using a jaguar dealer or an independent mechanic. Are there "better" gaskets I should acquire if it is a head gasket? How much should I be looking at for labor if it is a head gasket or intake manifold? Also, if it is a head gasket problem is this an issue where the heads need to re-machined to make sure it does not happen again? Anyone have this problem before or know if it is a known Jaguar issue with this V8 engine? Is there a simple coolant system check that can be performed to isolate the problem and make sure this doesn't become a trial and error mechanics lottery in labor charges?