It could just be a matter of bad connections or line noise…
Try bypassing the Netgear anyway.

Home routers tend to poop out with heavy traffic, showing
all the symptoms you described. The weak point is the
miniature motherboard of those routers. They don't pack a
lot of RAM in the first place and the buffers have to share
with all that software. P2P traffic is messy with small
packets and UDP chatter as well as long files and it's the
gateway's job to buffer parts and deliver packets once all
the parts come in.

Needless to say, a cup can only hold a cup. How much can
your computer hold? Think of it as a big, bad, industrial
strength Netgear, and you have absolute control over it.

Add an old 10 or 100 Mbit card to your computer and hook
that card to the modem. Just take the plug out of the
Netgear's WAN port and leave that hole empty. That wire
belongs to your new card now. Leave your original line to
the Netgear to keep it on the air. Set the TCP on all the
computers to use your 192.168.x.x internal IP address as
the "Gateway" and you're set.

If you use the Windows computer it's pretty easy to set
yourself to gateway mode, and you can do all the
portforwarding right in Windows. Any linux box can do
that, of course, but if your heaviest traffic comes from the
Windows box, that's the one you have to use.

The Netgear is now in your digital back yard and all it has
to do is glue all the computers together. This it does perfectly,
and everyone gets full speed access to the internet.

…of course, it could still just be a matter of bad connections or line noise… ;]