1: Do you have the same problem with a wired connection?
2: Try another channel (all chanels 1 by 1) (Change in router setup; wireless)
I also have a WL-138G in one of my workstations, and only get a stable connection on one specific channel. You may also consider changing to a WL-160g (If you're running W2K or WXp) This one gives me stable 54 Mbps connection irrespective of the channel with a number of conrete walls and floors between router (2nd floor) and workstation at the same positon as the WL-138g (ground floor). With the WL-138g I only get 24 - 36 Mbps connection.
3:Set preamble to short for the WL-138g (one of the options in the advanced tab of the WL-138G control center)
If this doesn't work please post a detailed setup including alle relevant settings on your router to get a sensible answer