If you have the money then I don't see why is it stopping you from upgrading your GPU, since it will also improve on how your computer handles software that handles heavy graphics.
If you like make that this is not a software problem, here are some things that you can do.
- Make sure that your VS X4 is updated - http://www.corel.com/corel/pages/index. ... id=1100002
- Try VS X5, the trial is more than enough to tell you if the problem is with the software - http://pnoy.me/coreltrials
- If you have other video editing software other than VS X4, you can try importing the HD video and check if the same thing happens.
- Check if there are other software running in the background that's causing conflict with VS X4 and the problem that you are having.
From the reply of other users where it looks like they're not having the same issue when editing HD video, I myself cannot recreate your problem and my laptop is less powerful than the one that you are using.