Don't know if your problem still persists, but as my post above indicates I too was experiencing a similar issue. But it now has disappeared and I can state definitively why.
Just recently I had to switch from AT&T "legacy DSL" to AT&T U-verse. While it is also a faster service, it is also always "on". When I wake my iMac the Internet connection is up and running, unlike the situation with my old DSL.
I can't say why one version of an Internet service versus another would be different in this respect, but it is.
For what it's worth.
