Hi! i just ran into a similar situation today. mine is an alto 2005 vxr had got it second hand. apparently the car started off with weird heat issues. i got everything done i could think of, overhauled the head, the rings, got the radiator flushed, serviced cleaned, new gasket, replaced worn out pipes, tuning, coolant in short many days and many bucks.
But even today it takes just few minutes before water starts boiling like crazy. 2 different mechanics (one being an old reliable dude who revived my brothers car from the dead). And they have told me that there is no use of trying to adjust the head further since the first owner of the car probably had his go at it several times (which was evident when they opened up the head first time in my ownership). Technically the abuse by the first owner is costing me a lot (this is my second car and I didn't have much choice other than this old car but that is another story). In the end both mechanics have advised to get a new engine and radiator.
Suggestions? I'm totally lost here.