Track sync goes nuts

Recently i finished mixing a track in Cubase 7.5 consisting of about 30 audio tracks. After a computer reboot i noticed that audio tracks at random where completely out of sync… Which is pretty wierd…

I use a couple of hardware synths in my productions, and i have experienced similar issues when my Access Virus TI was on when i was working in a project where it was not used.

Any suggestions what is causing this?