1998 Oldsmobile Silhouette head gasket?

Engine Cooling problem
1998 Oldsmobile Silhouette 6 cyl Automatic 108000 miles

i recently bought a 98 oldsmobile sillouhette (108,000 miles) the temp. Gage was bouncing from 1/4 all the way up to the red and back down again but didnt overheat however the other day it went from 1/4 all the way to the red and overheated and this happened a few more times on my way home.I took it in and they replaced a hose for the radiator that was leaking. When I picked it up I tried to take it for a ride the next day and again it went from 1/4 to 3/4 and I took it staight home and parked it. Could this be a thermostat problem or a head gasket problem?How much am I looking at spending for a head gasket?