This is certainly not usual behavior. Re-using a project file which may have actual data may be causing the issue.

A couple of suggestions:

• Ensure you are up to date on all service packs and cumulative updates.
• Try repairing any corruption in the file. Save as XML and then import the data.

Again, working with spare Cost fields should not have any effect on the schedule. Do you see date/time data changing?

If you open the file, press F9 to force a recalculation and then copy/paste do you see any changes?