Well I'm reluctant to post because the people in this thread know a lot more than I do.. But I figure when you're out of ideas anything might help. After researching this out of boredom here are a few things I have read..
-Head bolts are stretching under boost, causing combustion gasses to put pressure into the cooling system, dumping coolant out the overflow tank. By the way, a lot of the comments I read were from sti owners, so I'm not sure if some of the things below could apply to LGT's. Apparently this is a very very common problem with them, yet, there really isn't a clear cut solution from what I've read.
-Some say bad head gasket, and a new one fixed the problem
-Others say they tore the engine apart, head gasket fine, not the problem.
-Mixed up cooling system lines. The line that should be going to the top of the radiator was going to the overflow tank, and vice versa.
-Air in the system, or air entering the system somewhere, as previously mentioned.