Sounds like there's a summary field there somewhere. Could that be the case? Maybe with calculation fields that access the value of a summary field. What you describe is typical behavior for a summary field--they go blank when you open a record for editing. This isn't really avoidable unless you find a way to use an aggregate function instead of a summary field.
Not directly a summary field, but a calculation I'm using in the portal relies on a calculation which is derived from a summary (summary maximum) field. I'd guess this to be the root of the problem.