Hi Jo,
Age old problem this one. Having spent the last several weeks looking at various issues surrounding audio and video - I'd have to say that it's the encoding and codecs used that caused this problem. I thought originally that it was limited to WMV but even MPGs crash. I found that Sony Vegas and the Mainactor codec produced the most reliable files that could be paused, rewound and fastforwarded but that the tradeoff was that the files were larger.
Annoyingly, Quicktime files handled really well even at smaller (better compression) sizes. There are lots of posts relating to the best format if you do a search.
If you can post some more information about the videos or allow us to look at one, then we might be able to help.
Mack
P.S welcome to the world of Opus