howdy,
there are a number of things it could be,.
have you recently had anywork to the car done where the water system was touched?
there could be a loose/broken pipe,are you loosing water,does the car overheat,does your fan kick in at the correct temp,do you have warm air from the blower,..
is it pinking?the head gasket may have gone but also the head could have cracked aswell.only a pressure check would pick this up though.also if your water pump has failed and i have seen it before trust me...then that could be the problem,not sending the water round the engine,.
i would bleed the water system up correctly first off all.
my old 1.4 did this every 3 months or so,id bleed it up and then 3 months later it would do the same,but never lost water,.
ok bud that lot should keep you busy.lol
cheers
big al