Internet connection slowing down

I connect to the internet via a Belkin router and this normally works OK.
Occasionally I use Flight Simulator on line ("gaming") and after about half
an hour the internet connection seems to be grinding to a halt. Various
parallel internet services services drop out (2 voice channels, other
players positions, weather updates etc). Services running within the
computer continue to be fine. If I restart the router everything comes back
to normal (by which time I've messed the game up). The ISP as usual says not
a problem their end and it must be my PC.
Can anyone help with a) Are there any utilities I can run which can give me
a clue as to what is actually happening, do I need to understand things like
"bandwidth" and "packet loss" and b) any ideas what the problem is?
TIA
Barry

AsusP4C800-E DL
P4 2.8
1Gb RAM
ATI 9800 Pro
Dual SATA RAID
Belkin Broadband modem/router