Running Directx causes ADSL Modem to disconnect

I've just noticed that whenever I run FS2002 I lose my ADSL connection (the pair of black/green lights in my system tray turn red and I'm disconnected from my ISP). Stopping FS2002 restores connection. Same thing happens with Earthviewer when I get to a certain point. There's obviously a clash somewhere but everything checks out ok in device manager although I see that the modem and the graphics card geforce 3 ti 200 share IRQ 11. Running Windows XP, Directx 8.1, Athlon 1800xp. Variety of Nvidia drivers but makes no difference. Disabling anti-virus and firewall software makes no difference. I can also force it to disconnect by running the Directx tests in DXDIAG - connection is dropped when running Test Directx 8 interfaces. The modem is made by DLINK and I'm using their latest drivers. Has anyone any ideas whats going wrong?