Export to saved location - intermittent failure

I'd love to get clarity on some irritating issues (inconsistencies/bugs) with Filemaker's handling of imports/exports:

1) I have a script that saves an export to C:Mergemerge.mer (using specify file in the export script) - and it sometimes fails to export to that location, choosing, instead, the Filemaker Pro directory. The solution is hosted by FMS 5.5 and Win2000, and all workstations are Win2000, running FMP 6 with recent updates. It will sometimes fail on one workstation but will work just fine on another! I'm sure this has to do with the lazy way Filemaker records an export path - does anyone know exactly how it does this and how to avoid it failing?

2) import mapping - If I save a custom import mapping order and then create new fields in the source or destination file, do I then need to re-save the custom mapping order? Again, how on earth does Filemaker store custom mapping - is it by the field name (e.g. the unique ID each field is assigned) or something more prone to failure? After delivering updates to solutions, I've had clients show me how certain scripts that import records from other files no longer map correctly! Very frustrating - do I have to go back over each import script each time I add fields...?

Thank you!

Chris