How Can I improve My Wireless to connection for Linksys wrt54gl and Palm?

In our company we use a Linksys WRT54GL Wireless-G router 2.4GHz.

The router provide a wireless medium between 3 Janam Palm XP30 and our main POS unit (Server). The wireless router is configure for MAC filter and has a Secure encription key. The distance between the router and the devices at any giving time does not past 40′ feet. It’s an outdoor enviroment. The problem am having is that the device keep losing connection to the AP and once restarted, I have a difficult time making them reconnect. Eventually they connect but this should not be happening… I will appreciate any advice. Thanks