Try to determine if it's the access point, or your PC:

Access Point: Use a utility like NetStumbler to observe the signal strength. If you see it dropping off after two minutes, like you describe, something is affecting the signal (interference from other RF devices, etc.). Easiest way to address this is to just change channels, and try a new one. Try a few different channels.

PC: Look in the device driver, or power settings for the WiFi card, see if anything is set to 'adjust power', or 'allow automatic radio control' or something similar sounding. If it's always happening two minutes after you establish a connection, this is the primary suspect.