Hi, I installed Draytek 2700G with my ISP and everything is working fine on a cable but if I want to use wifi my router is drooping wifi connection every 2-3 mins and reconnecting after about 30 sec. Any ideas why it's happening? I have got newest firmware (2.8.3) and tested in on 4 computers.

Sounds like interference form nearby networks. Try using netstumbler or similar to see what cahnnels are in use in your neighbourhood and pick a channel as far away as possible from the rest.