Is there any particular action or section of the site that seems most error-prone? Also, is there a particular time of day or day of the week that you've noticed these happening? Do you have a way to check usage and/or load to see if there's any correlation with the error occurring v. the number of users?

Would modifying the code to write to a log or send an email be a possibility? You may not notice a helpful pattern just based on user reports (as not every user will report every error) but a log of every occurrence might make the cause and/or solution more obvious.