Computer freeze after some time idle with screen turned off, AMD proprietary driver

[Guest]

Hi guys!
I've installed Kubuntu 14.10, migrating from Opensuse, but I have huge problem, when I install proprietary Radeon drivers, every time after screen automatically turns off (after 10 minutes of inactivity) and I leave it like that for some time, everything completely freezes:

• I Can't switch to Virtual Console CTRL+ALT+Fx
• Screen stays off
• Pressing Power button does nothing, but a HDD Led lights up for a moment (I set option to Shutdown when power button is pressed)
• Only option is hard reset
• If I turn off Screen Energy Saving, everything works fine indefinitely
• If I switch back to opensource driver, everything also works fine
• Suspend is disabled in kde settings

I use:

• Radeon HD 6870
• ASrock MB with AMD Athlon II CPU
• 64 bit Kubuntu 14.10
• Maybe this also matters: Kubuntu is on old Maxtor pata drive, because it's test install
• I'm as sure as one can be that hardware is not the problem, as everything worked on Opensuse with proprietary drivers (and Win7 also). The only new thing I've added is that PATA hdd, but that shouldn't be the problem, it was working fine in another box.
• Seasonic 620W PSU

I couldn't find anything interesting in syslog expect cron hourly runs sometimes in the timeframe from screen off to computer freeze, but couldn't find any connection with problem yet.
I would appreciate any idea/help! I was looking forward to switching to Kubuntu, and everything was great until I noticed this.
Thanks!