If your electrics are playing up it may be worth checking the fuses on top of the battery.
Pictured here:
May not be the one pointed out here, was a picture off another forum!
Symptons:
Juddering, cutting out, struggling to use electrics, slow wipers, dimming lights etc.
Fix:
A new fuse holder on top of the battery, Part number: 1JO 937 617 D Around £70 from VW.
Also worth clearing fault codes after as all the ABS and airbag ones will be present due to them not getting enough voltage. I'd advise getting it sorted ASAP as mine cut out on the motirway and needed towing in the end and it may kill your alternator or/and battery!
To check the battery:
Charge the battery or use jump leads to get it running. Then use a multimeter on the batt when its running should be 14v + (quote by Richw911 from original threead that's linked at the bottom of this post)
What happened to me (some useful info in there)
http://www.golfgtiforum.co.uk/index.php?topic=89101.0Any problems PM me and I may be able to help you!