mystery file issue, import from maya

[super gnu]

ive got a load of heavy (4gb+) maya files, which im bringing into max via fbx. the models are very nice and the process is quite painless.

however a couple of the heaviest models have a strange issue.

import, save as max file.

next time i open , "error loading" message, hit ok

warning "obsolete data format , please resave"

file opens fine

pretty unstable with a habit of crashing on render end

there are about 10,000 object in the scene, all linked n various ways i dont need.

so to clean file, unlink all geometry, delete all helpers and splines (dont need them)

select all geometry, save selected.

merge into new scene, same error, but model comes in ok

same instability

repeat process, but this time grouping all geometry into a single group before save selected.

merge, and in the object selection i have my group, and an unnamed object at the top.

if i import just the group, it comes in with lots of parts missing, or moved, but no errors.

if i import both, i get the errors, and instability, but model comes in correct.

if i import just the unnamed object, i get the errors, but an empty scene.

in original max file i dont see the unnamed object.

tried re-exporting as fbx, and re-importing, but max crashes on import.

bearing in mind i have repeatedly unlinked all geometry, reset transforms, collapsed all objects to poly objects... im stuck. no idea how to fix it.