Ethernet reset/disconnect on wake-up

When I wake up my iMac the Ethernet connection disconnects (or perhaps resets)

Let me explain what I'm experiencing:

My Mac is sleeping (Selected the "Sleep" option from the apple menu in the top-left corner) and while it is sleeping the ethernet link is active (the light is lid/blinking on the switch it is connected to) se picture.

But as soon as I wake up the iMac, by pressing a key on the keyboard. The light shuts off, and I have to wait about 30 secs. to 1 minute. before it becomes active again.

This causes all sorts of problems afterworths.

• My PC is joined to Active Directory and uses Mobile accounts. Problem is sometimes I'm not able to login and I get a little red dot in the top right of the login/lock screen telling me that network accouts are not available. And even worse I'm not able to login with a local admin account either.
• It seems that the SMB/CIFS client also stops working thus not being able to connect to any network shares on my NAS.

The only way to resolve these issues is to reboot the iMac, and I'm not able to do that cleanly, I have to hold down the power button, to resolve that.

I hope you have any ideas on how to fix this (perhaps an apple tech could shed some light on this)

System info:

Model Name: iMac

Model Identifier: iMac15,1

Processor Name: Quad-Core Intel Core i5

Processor Speed: 3,3 GHz

Number of Processors: 1

Total Number of Cores: 4

L2 Cache (per Core): 256 KB

L3 Cache: 6 MB

Memory: 24 GB

Boot ROM Version: 231.0.0.0.0

SMC Version (system): 2.22f16

Serial Number (system): C02P7BN2FY10

Hardware UUID: 6A29D4E0-DC3C-5DD3-BCC8-BA925312B27A

Thankyou

Michael