Macbook Air, Leopard & VPN

I've been using a Macbook Air with a VPN connection that is perfectly stable on my Windows machine, but which disconnects after anywhere between 3 and 10 minutes on the Macbook. I've had an array of messages in system.log, varying from the "dlil_output: output error retval = 37" error, to just simply being disconnected.

My router is a Draytek 2800 series, my Macbook is barely out of the box and fully up to date and my PC is a WinXP system, also up to date.

Any help would be greatly appreciated.