I think I'll stick to F4 for now,the rig is where I want it to be and stays there stable.
"if it ain't broke, don't fix it" applies here too as in most cases in daily life!
As for the backup BIOS switch you're right,the CMOS_SW just restores everything back to default although not to default BIOS but rather to default settings.It uses the latest installed BIOS every time,not the one the board was originally purchased with.
Thanks also to ex58 for pointing out the news about F5 release.As I don't have good memories from that one I think I'll skip the "5" series updates and wait to try the next one F6 whenever comes out.
Many thanks again shadowsports,your help is appreciated!