Fields not being saved on customized Sales Order form

I have a client who has a fairly heavily modified SO form (although we cut back from what we had previously done by quite a bit).

The client will have an issue with the Freight amount not saving on the Total tab.

I will go in and recreate the customized form, and it will work for a while.

But, alas, the issue always comes back, and for multiple users (all of whom would be accessing the same All Company/All Users form).

Last time it only lasted a couple of weeks.

Has anyone else suffered from this, and do you have any suggestions?

(When it was more heavily modified, the field that wouldn't save was the Header Comment field.  Go figure.)