You are either buying duff belts or there is a problem with the pulleys there is no other reason for the fault to occur.You say there is not a problem with the pulleys but have you checked them under load at 4000rpm for which you need a rolling road I think maybe you will find an alternator clutch is failing.

Have you had any component changed that's driven by the belt. Alternator perhaps?

To strobe a belt you need to paint or mark a white line - preferably in the centre of the belt - for about a metre in length - all round is best... Got to be dead straight.

Then apply a strobe (timing light)) to the belt at various places. and look for the white line travelling sideways. Best done in a darkened garage , or in eveing or bonnet partially shrouded so you can see better.. depends on the power of the strobe light.

If there is no deviation, the line should stay in the same place..

I agree with the above diagnostics.. Soemthing is either loose or worn and the pulleys rock under load.. You should see it happen if you rev the engine from idle as maximum load occurs on changes in speed.

I would guess crankshaft pulley, idler pulley or water pump... but any component can be at fault..

Modern alternators have a clutch to avoid stress and cut out excess driving speed.

Edited by madf on 24/07/2011 at 10:31