Wireless Internet Cuts Out - Have to do on/off with Router

[Finchy]

I've been having a problem for a long time now, and im getting sick of it
I'll be on the internet when suddenly ill lose connection, the PC says in connected but nothing works.
It usually happens when doing several internet useing things and/or several Internet explorer windows, but sometimes it'll happen whatever i'm doing.
Sometimes it won't happen for a few days, then it might happen 4 times in an evening.

I have a Dlink DSL- 604+ Router and an Airplus DWL-520+ PCI Adapter

We have Virgin Plan 1 Broadband up to 8meg, but at best its about 4meg
Also, sometimes it reaches 4meg speeds but a lot of the time it only downloads at 10x broadband speeds

Thanks in advance for your help
Adui- Finchy