What could be wrong when a 1999 Monte Carlo overheats and loses coolant?

My first guess is the intake manifold gasket. The best bet is to pressure test the system and see where the leak is/.

so where's the coolant going? down and out the tailpipe? in a pool under the waterpump, towards the transmission? not visibly anywhere, but simply dissapears with use?

One common cause of overheating and coolant loss is a bad coolant bottle cap. If it doesn't seal, it takes a lower temperature to boil the coolant. Tightening too hard or too often can damage the gasket and/or warp the bottle. --- check it out before spending a lot of time and money on tests.

If your not seeing a pool or it leaking, as already mentioned its possibly the gasket or a cracked head (as already mentioned) or even a bad cylinder wall. My car had the same dilemma, it was actually a cracked block, decided not to bother fixing it and just sold it off as is.

and RandyS is where now?...busy thumbin' it-