So, as I'm scrolling through the bug reports section, I'm seeing a lot of threads about having issues connecting to the server. Before you make a thread about this, make sure you have a crash report. If you ever, ever, ever, see the key words: Failed to write core dump. STOP RIGHT THERE. There is an easy fix to this issue. Go into a single player world or go to 1.8. Click Escape ---> Options ------> Video Settings -----> VBOs On ----> VSync On If you are still having issues, then submit a bug report. However, VBOs is a lifesaver as sometimes even Optifine can't save you.