One thing which occurs is that your friends' laptops may be connecting in a different mode to your own. In the router's wireless settings, there will be various options such as 'b only', 'g only' or 'b + g' - I don't recall exactly what terms the DG834GT uses. Perhaps if you connect in mode g and they connect in mode b or something like that, perhaps this confuses the router, although it shouldn't do.
You could consider upgrading the firmware to the most recent DGTeam version, which you can get
here.
