You won't believe me...problem solved!
After reading all day long nVidia GPU forums I've solved the problem with that steps:
1.Downloaded and installed 314.22-notebook-win8-win7-64bit-international-whql.exe ( the most stable driver for my 640M)
2.Overclocked the GPU just a little but like +50mhz (this will allow the 640M to operate everytime in burst mode)
4.Frame Limiter:58 FPS
3.Vsync Off
5.Pre Rendered Frame: 1
6.Power Managment: Maximum Performance
Game is super smooth and responsive
1 & 2 are the key from what I've read..BTW seems like nVidia Optimus is just broken in terms of stability...forums is full of complaint messages.
Thanks for your support skoadam!