Wouldn't you know it. About 20 mins or so after my last post, it started playing up again. I had another look at the SAF page again, and noticed that it recommended using 1.1.0 alpha. I installed that, and it's actually been much better.
The volume is still lower than if I have the decoder connected direct to the television, but the picture is much more stable.
I'm not sure how it's going to record to DVD, but it even seems to be recognising widescreen transmissions better.
Thanks again for your help. I appreciate it. I might eventually have worked my way through all the information available and discovered SAF for myself, but you definately saved me alot of time.
I spoke too soon. The problem has started happening with 1.1.0 as well. A difficulty is that I don't know of anyone who is using this hardware configuration either with MediaPortal, or with any other programme, so it's hard to know where the fault lies.