Random restart when powering up

[red eye]

Wondering if this is normal for asus boards or something wrong?
Currently I have a z77 sabertooth, i5 2500k @ 4.5ghz 1.3.5v. Sometimes when I power up it shuts down and powers up as normal. It's been like this for the three years I have had it my previous asus board did it aswell which now has an i3 in it running openelec and boots up first time every time. What's going on?

Edit running a seasonic x850, previously antec tru power 750 which is now in the htpc

 

[Davedree]

My 2600k used to do this from new and then it became worse if the pc wasn't used for a long time. My gigabyte board has a bug where the dynamic voltage (vid) can reduce below default and so my offset voltage ends up lower than it should be.
But even at stock speeds it would restart on powerup, increasing the qpi/vtt voltage and also a slight bump up on v-core has stopped my fault for over a year now.

 

[something daft already!!]

Have you checked for an updated BIOS?