I have AVG running
Do you have its Resident Shield feature enabled? I ask because at work a while back our PC would sometimes (rarely) not save a file when it should, and the saving app would crash. It turned out that the problem was having McAfee's virus scanner set up to scan files when they were read/written (which sounds like what AVG's resident shield does). Turning off that "feature" of the McAfee antivirus software stopped the problem.
This might not be applicable in your case (or with AVG) at all, but thought I'd mention it just in case.