coolant problems

Firstly Hello all !!

Ok I have a clio 1. Recently I have had a problem with the cooling system remaining pressurised even after the engine has cooled ( ie overnight). To repair this I replaced the expansion tank and cap. This worked fine...however I found that I still had a problem with coolant levels rising and falling large amounts....then out at the supermarket the other day I started it and was greated with clouds of white smoke...which cleared after about 2 miles. I took the car back home thinking HEAD GASKET but after talking with a breaker friend he suggested the white smoke could be an injector problem and the coolant fluctuations due to a dodgy thermostat.

I replaced the thermostat with another that I had checked opened in very hot , nearly boiling water. The old ones seal was in a terrible state and the old thermostat didnt appear to open properly. I also then added some injector cleaner to a full tank of diesel and went for a drive.

NO white smoke , even on starting after stopping however lots of frothy bubbles in the coolant ( not mayonaise more like washing up liquid bubbles) and the coolant level rose to fill the tank and exit through the pressure cap.

I thought ok I havn't purged the system right and sure enough found a fair bit of air in it, next outing same thing happened , I purged more air (repeat until bored)

THe white fumes do reappear on tickover ( but not often and only when the engine has been run for a bit) , but disappear after a little accelerator

there is no mayonaise in the oil or traces of oil in the coolant.

Anyone got any ideas?

Thanks in advance

Ratso