T520 boot problem

[libco]

Hi everybody, I have this problem: every about 3 days I get boot mgr is missing error if i use Windows boot manager or stuck at Grub loading if I use grub, It happens when i shut down or reboot from windows and also when Im not even installing any updates or what. I use default pre-installed W7 and Ubuntu next to it. What can be the problem? can i somehow lock that bootmgr? or what is the solution? Thank you all... its so annoying to reinstall boot manager 2 times per week