Editing midi in piano roll computer freezes

Ok, I am running Sonar X1 Expanded and am trying to write / edit midi notes in piano roll. It goes ok for a bit, but then the computer will freeze and I am forced to reboot the computer to get out of it. I am running an I7 computer with 16 gigs of Ram built by a professional audio builder, I should not be having trouble editing simple midi notes. I have Superior drummer going and maybe 4 tracks of audio. I start and stop a lot as I edit the midi and I think that triggers these freezes, but it seems odd as the computer has the horsepower to handle this? Latency is set at 256, running Echo layla 3G with all latest drivers. Anything I should look at?