Suzuki vitara over heating randomly?

[bodhi_Australian]

I own a 91 model suzuki vitara. Only sometimes the engine over heats, like i can go driving and everything is fine but then i can drive for 20 minutes and it over heats, it sprays all its water out of the over flow pipe. Its got a new water pump, the fan belt is perfect, it has new oil, i just flushed out the Radiatorr and put new coolant in it. I checked the oil and the oil isnt milky or anything and the no water is leaking from anywhere. I have no idea whats wrong with it, anyone have any ideas?
Thanks.
sorry i should have added, when the car got a new water pump the mechanic removed the themostat and yeah i ran the hose through and the water seemed to come out well, i will retry it though just in case

[pacemaker]

try a new radiator cap , sounds like losing pressure form the cap

[Malcom M]

i agree with bake g. get pressure test done for leaks in the system.