WLAN dropping out on big file transfer

usually my WLAN chuggs along nicely all day on the internet and short file
transfers are no problem with about 10 Mbps.

However, if I transfer a bigger directory, it will drop out after about 2
minutes "Can't copy this file, as there is no connection" (or similar).

What can I do to have a constant transfer without the connection dropping
out? Is it a matter of throttling the router's max transfer rate, or can I
try changing the thresholds, and which ones, by how much ?
I'm transferring data from the wired PC to the wireless PC via router.
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My equipment:
2 desktops (1 wired, 1 wireless), one modem/router,
running wireless infrastructure mode.

Details:
PC 1 (wired) showing 100 Mbps connection in the taskbar icon.

Modem/router (wired&wireless, 54 Mbps) with the following settings:
max Tx rate: auto
Beacon interval: 100
RTS threshold: 2346
Frag threshold: 2346
DTIM: 1
preamble: short
Antenna TX : Full

PC 2 (wireless .11g set on WPA-PSK), Tx enabled: 48 Mbps.
The reception strength of this wireless unit shows up around 48-50 percent
on average (steady). Some brickwalls are in the way.

Thank you for any help
H.N.

Herbert Nagler