Windows 8.1 should update its drivers automaticaly, so I think the problem is elsewhere, more sounds like that the new driver have other default settings, and for example the Wake up over lan function is enabled by default.
If it only boot up again if the LAN cable is plugged in, maybe it waked up over lan by a wake up over lan package or because of some network activity (some card and it's drivers could make it, but I think you not have of this type of cards :) ). Try to disable the Wake up over LAN functions.
- Right Click on the Network icon on try
- Open Networ and Sharing Center
- Click on the left panel to the Change Adapter settings
- Select your LAN network (maybe it will called Ethernet)
- Right click on Properties (you need Administrator rights to open it)
- Click on Configure button
- Select Advanced (or Power Management) tab
- Search for Shutdown Wake-On-Lan, Wake on Magic Packet, Wake on pattern match
- Remember all of this default settings!!!
- Disable this three item
- Restart computer
- Try to sleep the computer after restart
If it is work and your's PC stay in sleep mode then your PC get Wake on Messages from somewhere on your network.
If in not stay in sleep mode set all the setting to the previous value.
2Sounds like you need to update the chipset drivers that support Windows 8.1 – Ramhound – 2013-11-07T13:46:05.317
Ramhound can be right. Actually, update all your drivers. – gronostaj – 2013-11-07T13:54:39.897