Why is the Coolant in reservoir boiling over.

[LOCASH]

I am not being at all sarcastic-could someone explain how a bad rad cap would cause my res. to fill with boiling coolant? I think it may happen when I really get on the engine. It happened once at the auto-x and it happened again today when I was trying to get away from the Italian Police. I saw them parked on the side of the road and it was really obvious that I was speeding. As soon as they pulled out I floored it for about 2-3 mins. to make it home. I drove the car to work and back (about 15 mins. one way) earlier the same day with no problems though. I'm sure you guys know more about this than me, but does it still sound like a head gasket? Thanks alot for all of your help. I really appreciate it.