More information would be useful:
Which version of Photoshop? Have you installed all the Photoshop updates?
Which OS version?
What sort of image are you working on?
Photoshop CS5. All updates installed.
What sort of images? Not file format: size, bit depth, color mode, content, layers, etc.
We need to know how to reproduce the crash that you are seeing.
Sizes vary between 22 - 35 M. Colour mode RGB. Bit depth 8 or 16. Content of images and number of layers vary.
It happens on flattened images as well as those with layers. It happens most with images I have been working on for quite some time but has started to happen with even newly opened images.
File size doesn't tell me much. How large are the documents, in pixels (width and height) ?
So far the only bug I can find with magnetic lasso is in documents over 65535 pixels wide, and I think I fixed that one before CS5.
About 3600 x 2640 pixels.
Hmm, I can't reproduce that.
I looked at the crash reports, and the crash is in a spot that's odd. I suspect something corrupted the stack before hitting the crash.
The good news is that in the next version, I have added more error checking in that area because of previous crash reports I couldn't explain.