Thanks for your reply. The complete source file properties are below, and I will look into your other suggestions next week. Don't know about codec packs, though. Can you explain?

Steve

General
Complete name : C:\Users\Steve\Desktop\project files\reencoded files 1280\MVI_8746.mov
Format : QuickTime
Format/Info : Original Apple specifications
File size : 103 MiB
Duration : 37s 720ms
Overall bit rate : 22.9 Mbps
Encoded date : UTC 2013-09-02 15:43:30
Tagged date : UTC 2013-09-02 15:45:05
Writing library : Apple QuickTime

Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : Baseline@L4.2
Format settings, CABAC : No
Format settings, ReFrames : 2 frames
Format settings, GOP : M=1, N=125
Codec ID : avc1
Codec ID/Info : Advanced Video Coding
Duration : 37s 720ms
Bit rate : 21.3 Mbps
Width : 1 280 pixels
Height : 720 pixels
Display aspect ratio : 16:9
Frame rate mode : Constant
Frame rate : 25.000 fps
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.925
Stream size : 95.9 MiB (93%)
Language : English
Encoded date : UTC 2013-09-02 15:43:30
Tagged date : UTC 2013-09-02 15:45:05
Color primaries : BT.709
Transfer characteristics : BT.709
Matrix coefficients : BT.709

Audio
ID : 2
Format : PCM
Format settings, Endianness : Big
Format settings, Sign : Signed
Codec ID : twos
Duration : 37s 720ms
Bit rate mode : Constant
Bit rate : 1 536 Kbps
Channel(s) : 2 channels
Sampling rate : 48.0 KHz
Bit depth : 16 bits
Stream size : 6.91 MiB (7%)
Language : English
Encoded date : UTC 2013-09-02 15:43:30
Tagged date : UTC 2013-09-02 15:45:05

Downloading the current version of Quicktime seems to have resolved the problem (so far...). Many thanks for the tip.

All the problems in my original posting have returned in the last 24 hours, after several days of problem-free editing since I downloaded Quicktime. In addition to the problems I listed I'm getting error messages when I try to render, saying my system memory is low (hard to believe with 16GB of RAM). I have tried freeing memory by turning down Dynamic RAM preview, reducing rendering threads to 1, reducing video thumbnails on the timeline, but no good. I have also followed advice to uninstall the program, create a new admin profile and reinstall, but that has not helped either. Tried the Vegas 12 trial, but I am unable to open my project with it. Help!

One of the most worrying things (belive it or not) is that the newer version of Movie Studio doesn't open your project.

Have you tried the following?

1. Select all events in your projects.
2. CTRL-C for copy
3. Open up a second instance of Movie Studio (and then set up project properties)
4. CTRL-V for paste all events into the new blank timeline?

One other thing to try is to update your video card driver.

Thanks Mark. I have updated my video card driver to the latest version. I have also tried dividing my project into parts 1 & 2 and I am back in business at least for now. (It still troubles me that my system apparently can't cope with the complete project, it's 33 minutes long and not that complex.) If it goes wrong again I will try your other idea of pasting it all into a new timeline.

When I said it wouldn't open in version 12 of Movie Studio, I should explain that it opened the project but would not load all the media.

If you are trying MSP 12 64-bit version, remember this
64-bit won't show your 3rd party 32-bit plug-ins, 32-bit should
You can have both installed (and of course keep v11).
In v11 breaking up a large project into smaller pieces should help, but v12 64-bit has added 64 bit support which dramatically improves support for projects requiring large amounts of system memory.

The reason I suggested the new project is that if the project file has become corrupted, putting in a new project file should help.

Again, Movie Studio 12 should open up all of the media as V11 - unless there is another issue in the background.

Thanks, everyone, for advice. I succeeded in completing the project in V11 by dividing it in 2, and will now install V12 for future projects.

Steve,
This sounds exactly like a crash bug I fixed for Vegas Pro 12 this summer. We had a flaw copying data from the 32-bit plugins to the main 64-bit Vegas app. It only happened after a long editing session or a huge project, so it was hard to find. Unfortunately, the fix was too deep to translate back to Movie Studio 12. As you found, splitting your project in two is a good workaround. A less palatable workaround would be to transcode your QuickTime media to something we support natively, like MP4 AVC. 32-bit Movie Studio does not have the bug.

That's very interesting to know. I myself have been working on a long project (42 minutes) in VMS12 on a Windows 8 64bit low spec machine without issues.
I do need to transcode my Nikon mov files with Lightroom in order to get fluent playback in VMS. The files stay mov-files, but the properties change from 'quicktime film' to 'Mp4-movie'. I guess Lightroom must be doing something to them which Vegas likes...