Golf 93 intermitent over heating

HI all,
Please help; I have a golf 93 that over heats at random times:banghead: , I can put the car through it's paces trying to make it over heat yet it refuses or I can be on a normal drive and it sudenly over heats. The only slight consistency that I have with it is after a medium to long drive , say 2 hrs, it will probebly boil over after i stop about 30% of the time. I have replaced the thermostat, used rad-clean to flush the water system and checked both the fan and switch. My next line of thought would be, A, a blown head gasket; but can't see any evidence of water in my oil or gasses creeping up in to my radiator resevour, or B, a faulty pump, but i've never heard of a intermitent water pump.

Any thoughts or advise will be greatly recieved.

Thanks!