External References - box unchecks itself

Under System Options, External References, the checkbox next to "Search file locations for external references" has started to uncheck itself quite frequently here lately. When this happens, the assembly will open but some or all (depends on where the files are located) of the components will be suppressed and SW says it cannot find the file referenced. It can be fixed easily enough by checking the box again, but I'd like to stop it from unchecking itself.

Does anyone else have this problem or do you know what causes it?

Thanks.