Volvo Repair: Coolant Loss, volvo 850 glt, coolant loss

Question
Hello I have a 1996 Volvo 850 GLT with 168,000 miles on it. I
just purchased the car and I am having some problems with it
already. The biggest problem i am having is that I keep losing
my coolant. I can go for a couple of days with it just fine and
then the coolant gauge comes on and i have to pull over. When I
look under the hood the coolant is gone. I then have to wait for
it to cool and fill the coolant back up. Once I do this the gage
goes back to normal and the coolant light goes away. I have to
repeat this every several weeks but, when it is parked I never see
any liquid. I even parked it for 2 weeks straight and saw nothing
leaking out. I have also noticed that when I drive sometimes the
temp gauge just goes from normal to cold like it is tired of
working and then it comes back after a while and works fine. I
took this to the dealer and they did a compretion test on it
because they feared it might be the master cylinder but, it
passed that test. Do you have any ideas?  

Answer
you have leak somewere  do you use coolant or just watter ?you can have bad hadgasket if you dont have any leaks but first take it to volvo dealer pressureize the cooling system with pump no leaks than let the check out headgasket and also radiator maybe all you coolant is going in tramsmission if the radiator is broken internaly you can see on trasmission dipstick what color it is if milky  than it leaks from radiator this only applys if your car automatic tranmission  do you have any smoke from car when you drive  like white smoke?