It could be your card but I'd bet your processor is to weak. Trying to edit HD footage with less than a 3.0 Ghz clock speed is pushing it. I had an intel i7 2.8 GHZ and it crashed constantly. When I went to an i7 4770K it all stopped. When I first edited 4K-64mb/s mp4 video, the GPU couldn't handle it for previews but stabilizing it was OK.
My next step - dual quad-core zeon. This has been the benchmark for AVID certified machines for years.