auxiliary belt causing low revs?

[justin31]

ive got a problem with my 1.6 1997 passat.basicly when im driving have to stop or slow right down and i press the clutch in the revs drop right down to 400 RPM and then shoot back up to 900 RPM (idle speed)

ive noticed that when its wet my auxiliary belt squeels on start up for a few minutes and i only seem to get the revs issue aswell.

could these problmes be linked?

also my car has a automatic tensioner so im guessing i cant tension it anymore? the belt itself looks in good condition though

thanks