Cubase 8 pro freeze


i am using intel I7, 12G ram, Win7 64bit latest update.
M-Audio fast track ultra package 6.1.10 firmware 1.5.1
Cubase Pro v8.0.20 b468 64bit

when i work on project using instruments sometime it freeze and cant do anything
so i always end task, that will take long time because in still stack in my processes or restart my pc. this happen when i using something in background or i leave pc for few min.
i thought it was VST but no when i use cubase VST freeze again.

kindly advice