Volvo Coolant

I have a 1996 Volvo 960 Station Wagon with 137,000 miles on it. Great car! However, I have had trouble keeping coolant in the coolant reservoir lately. If I fill it to the fill mark, and run it like 10 miles, almost all the coolant is gone and I have to refill it. It disappears fastest if I stop and start the car frequently. If I fill it again and let it sit like overnight, a lot of the coolant is gone. I have checked the reservoir itself and it does not have a leak--even ordered a new one which should arrive soon--the hose going to the radiator doesn't leak, and there is no sign of the radiator itself leaking if you look under the car on the ground. I am mystified. Where is all the coolant disappearing to and what could be wrong? I have had this car since April. It did seem to use a little coolant, but now it uses a lot. I have always used Prestone 50/50. Please help.