Weird problem with Belkin PCI card

This is driving me mad:

I have 3 PC's upstairs, all with PCI wireless network cards.

One has Vista HP & a BT Voyager card, 1 has XP Pro and a no-name cheapo
wless card, and 1 has XP Home and a brand new Belkin card.

I bought a new router (Netgear DG834PN RangeMax) specifically cos it had
MIMO and there for better coverage. It sits downstairs, on top of my Xbox
360 (which is wired to it).

The PC with the new Belkin card drops the WiFi connection quite a lot (every
hour or so). The other 2 work perfect, all the time.

What's weird is that when it *is* connected, it connects full speed (54Mb)
and says "Excellent" signal strength. When connected, it never drops below
54 and "Excellent". But then it just drops, disconnects. If I then click
"Refresh network list", it finds some other cunts wireless network
(presumably someone else in the street), but not mine. If I click "Refresh"
a few more times, eventually mine pops up, and it connects again, 54Mb,
"Excellent" signal strength.

All PC's are about the same distance away from the router, and all are in
the same sort of position (i.e. within a computer desk/unit, back against a
wall). All 3 use the same type of aerial, the "screw on, plastic rod" type.

Any ideas what could be causing this? Is it not weird that it never has
"poor" signal or anything?