@tommi19
-1-
what happens if you shutdown your Mac and restart, while making sure you un-check the marker at "Re-open windows when logging back in" ?
[ this will make sure you are doing a full reboot ]
do you have a stable WiFi/AirPort-connection now ?
-2-
what happens if you do a reinstall of the MacOSX 10.7.3 Combo Update ?
you can download it here :
http://support.apple.com/kb/DL1484
do you have a stable WiFi/AirPort-connection now ?
-3-
what happens if you restart your Mac with all bluetooth preriferals shut down / disabled (like your keyboard and mouse) ?
[ try using both a USB-keyboard & -mouse (or USB-radiofrequency wireless ones), and/or do not switch your bluetooth mouse & keyboard back on (using the small ON//OFF-switch on the bottom) before your Mac has completely started up, so wait for re-establishment of the WiFi/AirPort-connection also before switching on any bluetooth periferal ]
do you have a stable WiFi/AirPort-connection using this procedure ?
-4-
what happens if if you startup in 'Rescue Mode' ?
[ by pressing the 'R'-key during startup ]
do you have a stable WiFi/AirPort-connection in 'Rescue Mode' ?
If so, read through this thread again and try either one of the .kext-replacement solutions suggested by various people (including myself)...
[ if you make sure you'll finally have the .kext-file installed from either 10.7.2 or 10.7.3, all of these solutions are quite similar ]
Good Luck !
;-)