Lower the 'Unwrap Factor' number a small amount
in the conversion settings. (Mine is now set about point45 )
I always us the same exact 'Master' format when retrying a different conversion setting (In other words, don't try and figure the NEW best setting on a file that has been converted after/during the previous setting. Restore the File if needed and use a modified setting..
Calibre remembers the Last Conversion settings used on that book
(The Preference is the Default for books never converted (or not reset) ).Calibre also will hint
input format type (which may NOT be the one you started with and needs to be changed back to the 'Master')