SRAM XO rear derailleur issue

pots74

Hi all,
I'm having issues with my SRAM XO x10 mid cage rear derailleur. When pedaling my chain goes slack and then all of a sudden goes back to the normal tension. This constantly happens and i can't get the gears to work correctly. Perhaps a tension spring?? :?
Can anyone advise with this?

Many thanks.

The Rookie

For the chain to go slack when pedaling you aren't pulling it tight, that or you have some very very stiff links in the chain staying at the radius they came through the cassette at and forcing the top down.

Nothing to do with the rear Mech!