My car keeps cutting out and over reving.

I have an X reg Peugeot GTi. It recently had the head gasket patched up around 2 months ago, but since then it has developed more problems. It over revs whilst stationery and cuts out everytime I come to a halt as though it has stalled. Just today, I have noticed very small plumes of white smoke emitting from the exhaust. I have recently spent around £1100 on it. Any ideas of the cause of problem or potential solutions and possible costs would be greatly appreciated.
Many thanks
Nick