Wireless router behind TV?

I'm trying to help a friend set up a wireless router but we keep losing the
wireless connection. He's on Virgin Media Cable (ex-Telewest area) and the
cable guys left him a cable modem (I think it's a Scientific Atlanta, but
may be wrong) behind the TV in the living room at the front of the house -
they wouldn't install it upstairs, which is where he really wanted it.

Anyway, the main (desktop) computer is in the spare bedroom at the upstairs
back of the house (3-bed semi, built around 1965), so he asked me to get him
a wireless router and wireless adaptor, so I went with a Linksys WRT54GL
router and matching Linksys WUSB54GC USB adaptor.

I'd read good reports of the DD-WRT third-party firmware so I have to admit
that I didn't try it with the factory-default Linksys firmware at all, but
just flashed the router straight away with the DD-WRT firmware. The main
reason I did this was because the router leaves the factory with a default
and non-adjustable radio transmission power of 28mW but the DD-WRT firmware
allows adjustment of that, up to (but definitely not advised) 200mW. Given
its position of having to sit on the floor behind the TV, I tweaked it
from the default 28 to 70mW.

The adaptor in the desktop upstairs reports signal strength and quality to
be either "excellent" or "very good", (never dropping to either just "good"
or "poor") but it frequently loses the signal altogether - it seems to be
"all or nothing".

Anyone got any ideas what to do? Anyone else running a wireless router in
close proximity to a TV?

Cheers and TIA,

Dave