Sounds like you've got some disk corruption.
Thanks, Scott, what can I do about it? Some files work fine, others don't, and I don't see any difference in saving, handling, opening them, so it seems a bit random...
Disc corruption has little to do with Illustrator. It's a symptom of a failing or overfull hard drive in most cases. Or perhaps some other hardware issue. Bad RAM can cause corruption as well.
First thing to check is how much free space a hard drive has. If it's less than 20% the total drive size... you need to make room on the drive. That may solve the issue.
Next test would be to alter the RAM configuration by pulling out a stick if possible and testing to see if the issue is still present.
Next test would be to replace the hard drive.
Beyond that you would need to have a repair service look into the issue.
cheers, Luke, but I'm not worried about my ai-files but the pdfs...
thanks Scott, sounds promising & a rainy Sunday's activity - I'll check them all!