I think the DHCP lease time is a maybe good workaround but not the solution:
I caught the problem in the system.log
This was the last message before the sleep:
Oct 26 12:42:38 imac kernel[0]: Ethernet [AppleBCM5701Ethernet]: Link down on en0
....
....
Oct 26 13:01:01 imac kernel[0]: Wake reason: EHC2
...
...
Oct 26 13:01:04 imac kernel[0]: Ethernet [AppleBCM5701Ethernet]: Link up on en0, 1-Gigabit, Full-duplex, Symmetric flow-control, Debug [796d,2700,0c01,0200,cde1,3c00]
...
...
Oct 26 13:01:08 imac.intranet.ronaldo.nl configd[17]: network changed: v4(wc4:9.167.246.77, en0-:192.168.1.2) DNS! Proxy- SMB-
Oct 26 13:01:08 imac.intranet.ronaldo.nl mDNSResponderHelper[4775]: do_mDNSInterfaceAdvtIoctl: ioctl call SIOCGIFINFO_IN6 failed - error (22) Invalid argument <<<<<----- This is the one
This is absolutely a bug!!!
And then again:
ifconfig en0 down
sleep 10
ifconfig en0 up
solves the problem for this moment.
which is also found in the system.log
Oct 26 13:05:45 imac kernel[0]: Ethernet [AppleBCM5701Ethernet]: Link down on en0
Oct 26 13:05:46 imac.local configd[17]: network changed: v4(wc4:9.167.246.77, en0-:169.254.140.71) DNS+ Proxy- SMB-
...
...
Oct 26 13:05:59 imac kernel[0]: Ethernet [AppleBCM5701Ethernet]: Link up on en0, 1-Gigabit, Full-duplex, Symmetric flow-control, Debug [796d,2301,0c01,0200,cde1,3c00]
Oct 26 13:05:59 imac.local configd[17]: network changed: v4(en0+:192.168.1.2, wc4) DNS! Proxy+ SMB+
No entry of the mDNSResponderHelper now, but I noticed it is reported in the system.log regularly.
...
I beginning to suspect that the link takes too much time to come online (4 seconds in my case). Maybe not always but sometimes.
By the way setting the speed to manual did not help, proved my problem today.
Also the lease time can't be the issue if you have a manual ip. And I also have this set with manual ip adres.