Unity 4 pro not responding and lagging in scene

I am running unity3d pro v4 on Win 8.1 pro, using Asus G750J.

Often Unity just stops working and task manager shows not responding tag.

Also, unity sometimes lags a lot in scene view, especially when using demanding assets like easyRoads3D pro.

I have installed directX 9c like suggested in some other threads.

Please help