Thick borders between some rows cannot be removed

A fairly simple workbook created years ago insists on showing thick borders between some rows. The thick borders show only in Print preview. The file is not locked. There is no conditional formatting. I've tried the following to no avail:

Selected affected rows and cleared all formats, then re-added thin borders;

Deleted all affected rows and recreated new rows with newly applied borders;

Used border eraser which seems to work (they don't show in Print preview), but when I reapply the thin borders by using any border tool or format painter, those same borders are again thick.

The thick borders show up between every tenth row, so there must be something other than actual borders going on. Ideas??

Thanks.