Constant internet disconnections

Good morning,
I have an intermittent, yet very frustrating problem that I hope somebody can help me solve.

I have a home network consisting of:
Main pc - Vista - ethernet cable to router
Laptop - XP - wirless
Workstation 1- XP - wireless
Workstation 2- XP - wireless
XBox 360 - wireless
ISP - Telstra bigpond "Cable extreme"
Modem / Router - Telstra supplied Netgear.

The problem is that I suffer random internet disconnections. This can happen at anytime but seems to be more prevalent during high bandwith usage. (Gaming etc). It is also quite frequent (6 times over a 2 hour period last night)

The only fix is to either reboot the router or reset the adapter on the Main pc. Once I have performed an adapter reset, and I get the internet back on the main pc the other pc's and xbox appear to follow suit and their connections are restored.

Whenever I lose the internet on the main pc all other devices become disconnected as well.

The router status is interesting in that even though internet has been lost on all devices the router lights signify that it is still online. Therefore it appears to me that the router just kicks everyone from the network at times.

I can find no logical sequence to these disconnections and am at my wits end.

I have already performed the following:
1) Contacted Telstra on several occasions -they have sent techs out to check the cable etc .. no obvious faults. They have recently replaced the router - it was a motorola and now is a netgear. They are both the integrated cable modem, wireless router unit. The motorola was a sbg 900 and the netgear is some form of an 814. I am posting this from work and do not have the prefix handy.

2) Replaced the network card in the main pc and the wirless adapters on the worksations.

3) Setup appropriate firewall and portforwarding rules.

Any help would be much appreciated as I appear to be going around in circles now.

Thank you

Keith