Welcome to the forum.
First, I would look over this ARTICLE to see what info would be so very useful in helping troubleshoot the problem.
Next, how much defragmened, free-space do you have on your HDD?
Then, I would look at this ARTICLE, which starts with a checklist, to get PrE running smoothly, and then moves on to troubleshooting tips.
Good luck, and please let us know a bit more.
Meantime, I'd recommend using Windows Media files in the program.
It CAN work with them -- but they can also make the program extremely unstable, as you've experienced.
What model of camcorder is your original footage coming from? There may be a better, more efficient way to load your media.