I would first assume that the material textures are not lost, but merely aren't being displayed.

If you select one of these affected faces, does the 'entity info' dialog box show the correct front/reverse materials?

If you open a model, then close it, then re-open it, are different faces 'loosing' their material? (You did say random...)

Are the materials still showing in the Materials Browser, as 'In Model'?

Have you tried using, in the 'Model Info' dialog box, the 'Fix problems' button? ___________

These questions/suggestions are just 'shots in the dark.' You haven't really provided enough info to diagnose a problem. It boils down to whether you question SketchUp or the model. If you want someone else to try opening your model, you can post it here. (See the 'Files' link in the right-hand column of this page.)

If you post your model, let us know, and tell us the filename.

Regards, Taff