94 olds cutlass supreme over heating

hello- i have a 94 olds cutlass supreme. originally a 3.4, but had a 3.1 put in a few years ago. some of the accessory pieces are still for the 3.4 (alternator, radiator, etc.) a couple of weeks ago the temp gauge would go all the way up to 270 but immediately go down to below 200. i took it to my mechanic friends and they found a small leak in the radiator, so we put a new one in. also replaced the thermostat and did a tune up. now it heats up to 270 after a 10 minute drive and does not go back down. it also shoots the coolant from the radiator into the overflow tank. even when the gauge gets up to 270, the radiator is cold, and the upper and lower radiator hoses are warm to the touch but not hot. my mechanics and my mechanically inclined family are stumped, except for it maybe being a head gasket problem, but a compression test tested good. any ideas? we are getting desperate