qb pos v5 r13 Pro -- items disappear from reports

I have a client who is using pos v5 r13 Pro, on networked XP machines. They are also using QB Pro accounting.

Lately, when they try to run Inventory reports from the Item List, items have been disappearing at random from them whenever they Modify the report by adding/removing columns. No Filters are set on these reports, and the items can be made to reappear by editing them in the Item list (e.g., changing one field then immediately changing it back and hitting Save) and Refreshing. The reports show up fine when the report is first run, they just start messing up once columns are changed. They noted that remaining columns will display items that have zero QOH amounts in them, so it's not a matter of zero-qty's somewhow being filtered out (that, and the vanishing items have nonzero quantity values on them).

Any ideas?