What version of Optifine are you using? I use HD_U_C5 when I switched over to HD_U_C7, I was crashing about every 5-10 minutes and I also experienced increased lagg. (Which is why I don't use C6 either.) Try going to an older version of optifine and maybe see if that works. Also are you running Optifine solo or through forge? If you have an older pc forge itself might be causing the issue if your pc is running out of cpu when trying to process that game data the forge requires. Just a couple thoughts let me know if it helps!!!