Over heating and We don't know what else to try?!

Hi all, we could really use some help brainstorming if anyone is feeling up to it...

We got our hijet 1.3 in December and just driving it home it overheated and we had a coolant fluid explosion!!!
Since then we have done lots of reading online and are now fully aware this is a common problem but we are running out of ideas on how to fix it.
We have replaced the thermostat and radiator caps and have bled it the way you all mention to on here about 5 times now.
So each time we start off ok, hot air coming out of the heater all ok then all of a sudden the air will go cold and the temp gauge shoots up!!!
We've had the fan kick in in the past so that works (sometimes).  Quite often we will check the hose coming out of the radiator and it'll be ice cold so of course then the fan won't kick in. 

We think there is probably an air lock in the radiator which is causing the cold air and the cold hose but no matter how much we bleed it we always get the same out come?! After about 8km that is, so we really don't get very far.
We don't think the head gasket is gone as there's no other symptoms to suggest that.

If anyone could offer us anything else to try we would really appreciate it!!

P.s. this forum has been a gold mine of info just starting to get frustrated at our lack of luck so far.

Sound like a block with air or something, take the hoses of the radiator and totally flush it through till it runs clear, then may be flush the engine as well to see if any debris comes out. Refill with antifreeze, bleed fully and try again.

If you bleed it up to perfection, but after a drive its "spewing its bag out".... more than likely the damage is done and the head has to come off it.

But if you don't cure the root cause, it will only be some time before you go round in a circle.

So you have to be 100% cure there are no leaks.... I pressure test them with air over night. Only when they hold 1 bar of air over night do I see them as "water tight".

Good luck, you'll need it