Be sure you are running the latest version of Mac OS X ( get 10.7.2+)
Apply all available system and software updates available to your Mac from Apple menu > Software Update
Reset the WiFi Router
After you’re positive you’re on the newest version of OS X and have all software updates applied to the Mac in question, proceed with the solution:
Add a New Network Location & Renew DHCP Lease
Launch “System Preferences” from the Apple menu
Select the “Network” panel
Click on the “Location” pull down menu and choose “Edit Locations…”
Click on the + plus icon to add a new network location, give it a unique name, and click “Done”
Back at the Network panel with the newly created location selected, click on “Advanced” in the bottom right corner
Click on the “TCP/IP” tab
Click on “Renew DHCP Lease” and wait for the numbers on the left to repopulate, then click “OK”
Renew DHCP in Mac OS X Lion
You should now have a stable wireless connection under Mac OS X Lion 10.7.2.
For most users the wifi stability issue was resolved with installing the latest OS X Lion updates, but in some cases is seems that old network connection profiles brought the problem forward for to the newest OS update.