Problems with Linksys WAP54G

I have Linksys router, a Linksys cable modem and a Linksys wireless
access point.

I and my son have been occasionally using the wireless from my Pocket
PC and from his PC.

The wireless connection seems to become established ok but after a few
minutes is lost from both the PC and the pocket PC.

Anyone got any hints as to how I can investigate this.

The wireless is not encrypted but I am using MAC addresss filtering.
Paul (note that this email address is not read)