If your windscreen wiper is slow and/or freezes up during a wipe across the windscreen, you probably need a new wiper arm linkage
Costs just over fifty pounds to buy from Audi and can be fitted by someone with a little knowledge in about half an hour.
The seizing up of the wipers is a common fault on VAG cars. (was told by a few people from within Audi) The rubber seal around the two ends of the wiper linkage near to where it joins to the wiper arms wear out and allow water to ingress. This in turn causes it to rust and seize. Hence the slow movement or seizure of your wipers and puts strain on your wiper motor to get them moving.
You should be charged no more than 1 hours labour to do the job by the stealer or garage you choose to use.
Unfortunately this fault occured to my car recently. In fact it has been one of a few things recently that have needed replacing. Its obviously now getting to the age where things are starting to wear out (5 years old) through general wear and tear.
Other than than, its been a superb car.
If you are going to try and do it yourself, remove the plastic cover over the top part above the engine bay (below the windscreen) where the battery is located (on the A6). Then unscrew ECU box and gently move to one side. Do no disconnect. Undo the nuts on the wiper arms where it joins the wiper linkage and then the bolts holding the whole wiper linkage to the car. Remove the whole thing out. Replace with the new and put everything back together. Make sure to mark/check the wiper arms are in the right position on the windscreen before you tighten up nuts.
Job done. Ideally use a Haynes manual as it will give you a detailed step by step guide.
Hope this helps