peugeot 307 1.4 petrol abs fault (intermittent)?

Hi All, Need some help.
Had my 307 for about 2 years no probs until now.
Brakes go like they have an air lock in the brakes and then the abs light comes on and the centre screen on the dash sees abs fault, once the light comes on the brakes go fine and then once you turn the car off and then start it back up again the light goes out and then it starts all over again. what joy.
Any ideas id be greatfull.
Many Thanks.

Rich
January 2012