Aargh! I have an iMac running Lion and have to manually get online several times each hour. I get a notification that I am not connected to the internet, i then run "diagnostics' it says my system configuration has changed and suddenly green lights and I am up and running- how on earth can i stay on the wifi more consistently? Thanks!
This could possibly be a modem/router issue, I've had faulty routers before that drop out every 10 minutes and have to re-connect. You could try to connect to different wifi networks to test, to get a clearer picture of what is going on.
Try one or more of the following tests, If the iMac works as expected (no drop-outs) you will know if it is your modem/router.
It sounds like your Wi-Fi network is conflicting with a neighbour's network. You can view extended information about the channels you're using by holding down Option when you click on the Wi-Fi icon in the Menu Bar, then use this to choose a channel that is distant from your neighbours.
Some more detail here: Wi-Fi problems on mac - particularly the last para which really seems to be a problem with extended AirPort networks (i.e.: multiple Wi-Fi devices).