What causes the ABS warning light to intermittently display - Nissan Almera?

I have a Nissan Almera - 06 reg. There is an intermittent fault in that, occasionally, the ABS warning light comes on and, at the same time, the speedometer drops to zero and stays there. This usually lasts for a few minutes then recifies itself. There is no apparent loss of brake power, or any other performance issue, during this period. I highlighted this when I took the car for it's last service but the garage was unable to trace the cause of the fault.