Firstly, FreeBSD 11 has not yet been released. It has only just been branched from CURRENT to STABLE (and that label should not be interpreted an expectation of stability at this extremely early stage of the branch).
Secondly, you really need to give at least some basic details about your hardware. In particular, the exact model of your GPU is essential information for that sort of problem. Are you running the amd64 build, or some other build? What revision of 11 are you running (exact SVN revision number if it came from SVN head or stable/11, or the name of the snapshot (e.g. BETA1, etc)? BIOS+MBR, BIOS+GPT, or EFI+GPT?
If you are looking for a reliable and stable system, FreeBSD 10.3 is a much better choice at this point in time. If you want the bleeding edge, 11, but expect potentially serious bugs before it is formally released as 11.0. 11 has only just stopped being a developer playground (meaning that in the best possible way, and with complete respect to the developers).
