1998 Cadillac STS

I kept getting the message "check coolant" I would put some coolant in and within 2 weeks I would recieve the message again. Only once did the temp gage go anywhere near the red. It is just burning through the coolant and the oil (although the oil may just be b/c it is an older car). I am worried that I am going to have to shell out $10,000 for a new engine. Does this year have alot of problems with the engine coolant system? Does this problem sound familiar to you? Do you know what it may be?

Note** I do know that it is not the water pump and that the coolant is not leaking into the oil.