ABS ON MY VECTRA ?

HI I HAVE A VECTRA 2.L DIESEL AND SOMETIMES WHEN YOU PUT THE IGNITION ON THE LIGHT STAYS ON AND SOMETIMES IT GOES OFF.IF YOU START IT YOU CAN BE GOING ALONG AND THE ABS LIGHT COMES ON.WE HAVE HAD THE WHEELS OFF AND CLEANED THE SENSORS TTO THE WHEELS BUT ITS STILL HAPPENING CAN ANYONE HELP PLEASE.