No Wifi Networks after Sleep

I'm running Mavrics for a while and all has been good. But lately after long sleep on my Macbook Pro (17" early 11) no wifi networks are available on the interface list. When I restart, they re-appear (most of the time).

I've tried creating a new network location, turned wifi on and off, even deleated the preferences in the library, but no luck.

I'm an engineer and this has got me stumped.

Thanks in advance!