Coolant Loss

I need help diagnosing a coolant loss problem.
There is a loss of coolant while driving but no visible leaks anywhere. The engine runs great, no loss of power or gas mileage. The idle is a little rougher than it was many miles ago.
The car is a '98 GS w/ 165k mi. The problem has been going one for some time and may be getting a little worse.
I recently flushed the system and replaced the coolant w/ Prestone EM.
So long as I add some coolant to the recovery reservoir every couple hundred miles there's no problem. If it goes empty, the temp will run higher and I find that the radiator neeeds coolant. So far I've caught it before any overheating occured.
Any suggestions appreciated.

In the L67 SII 3800 engine the two most common reasons for unexplained coolant loss are the lower intake manifold gaskets or the two o-ring seals between the supercharger and lower intake manifold for the coolant passages to the throttle body.

Best to get it nipped in the bud as soon as possible.

Thanks bnaylor.
I am afraid that the problem is one of the ones you suggest.
Couple of questions if you will be so kind:
1. Is there a definitive test or indication that will make it fairly certain that is the problem before embarking on the work?
2. I'm guessing that in either case, both the SC O-rings and the LIM gaskets will get replaced. Are there other parts that are likely to need replacement or makes sense to do at the same time?
3. Because of time and facility limitation, I may elect not to do this myself. Do you have a guess as to what a garage will charge for this job?

You're welcome.

Before you replaced the coolant did you have any noticeable sludge in the coolant reservoir, in the radiator like at the filler neck and back of the radiator cap? Any brown milkshake on the dipstick or the oil filler cap?

Best test would be an oil analysis test since some coolant may not be burned 100% in combustion and may get past the piston rings into the crankcase. Cooling system leak down test is another that can be performed but sometimes may not indicate any direction to go.

Other parts should be replaced like fuel injector o-rings and the two plastic coolant elbows since the tensioner assembly must be removed. Consumables like anti-freeze, etc.

For LIM and UIM R&R in your case supercharger work can be expensive at $1000 and up with all parts. Best to get a few estimates. For first time DIY type work you are looking at a whole days job to do it right and with parts cleanup. Plus the silicone sealant used for the LIM port and end seal gasket corners needs at least 24 hours cure time.