I am using linksys cisco wag120n adsl2 modem router.When I start the router the internet speed seems great but after some time the speed drops miserably. When I again restart the router the speed seems great for few mins but again drops after sometime.It happens in both LAN and wireless mode

The firmware version of the router is 1.00.16. and I'm using airtel broadband connection(India)

Please suggest some way out.

I want to buy another adsl wireless modem router please suggest which is the best one?