Mk5 ABS and ECU warning light fault

My friend has recently bought a 2001 Polo S with the 1.4 16v engine in. He is having trouble with the ABS and engine management lihts coming on every now and again. The woman who owned it before it always got it serviced at VW and when they checked the car there was nothing wrong with it, so they just reset the lights and sent her on her way.

The car doesn't perform any differently when the lights are on, so its not a major issue, but just would be nice to sort. Also both lights are never on at the same time, its one or the other.

If anyones got an possible ideas as to what it might be causing the lights to come on randomly they would be much appreciated!

Cheers