I had the same issue with my wife's 00 GA GT 3.4
It somehow lost the coolant. We refilled the coolant...it lost it again. We could find anywhere it was leaking out and it wasn't making oil so I just refilled it again. It again lost the coolant, so I got mad about it, flushed the system, ran straight water in it for a hours, probably 50 miles or so, came home and flushed it again. Refilled it with the normal Green coolant instead of Dex-Cool.
Haven't had the problem since.
I don't know what the problem was, I don't know where the coolant was going but it wasn't going on the floor of the garage or the crank case and it wasn't inside the car...but in any case, since I flushed and refilled with green, I've had no more problems with it.