iMac doesn't automatically reconnect to wireless

Hello

i have an iMac middle 2007 with the latest Lion upgrade (10.7.4 11E53).

I use a Wireless connection, and I noticed that when my router restart my iMac doesn't connect automatically to its WLAN.

So, basically:

1. my iMac is connected to Wireless and Internet

2. the modem/router turns off

3. the modem/router turns on

4. iMac doesn't connect automatically (waves on wi-fi icon are grey)

I tried with resetting PRAM, deleting the entry of wireless on Key-Ring and re-saving it, read a lot of forums but i haven't found a solution.

It is an huge problem to me, i haven't that problem with my iPhone 4 and the modem/router is working correctly with others computers at home, so it's not a signal problem.

I can't explain why but my router sometimes is restarted at night, while i'm sleeping and my iMac should download gigabites of Legal stuff.

I just want that my iMac it works like my phone and any other pc 😀

Thanks for any suggest and - PLEASE - avoid pointless comments like "don't restart your router" 😀