Engine false overheating

I have a 2010 ford focus. A few weeks ago I notice
my heater was acting up. When I let my car warm
up in the driveway it was just blowing cold air,
however when I started driving down the road it
worked fine. I figured maybe my fluid levels were
low, not the case. My fluid was fine. About a week
later I was driving and I watched the temp gauge
go bezerk. It creeps up like it overheating, then
bounces back to normal, then bounces onto the
red. However, my car was not actually
overheating. I figured it was the thermostat, so had
it replaced. No joy. Still doing the same exact
thing. Heater isnt working right, and the temp
gauge jumps to the red after driving aroind the
block. I called a local garage and they immediately
said head gasket. I'm thinking bull.