[Gimp-user] error animated Gif

[fvn18 <forums gimpusers com>]

i made a animated Gif (Gimp 2.8) but there is something wrong.

i made an escalator with 2 persons on it, they move up the stairs in 12 steps.
i made 12 different .XCF's for every step one.

Before converting to animated gif i make the XCF's 1 layer each.
when i use the option FILTER > ANIMATION > OPTIMIZE (difference)
the animated gif leaves black lines when the persons move (see picture
attachment) the export file is very small so that's ok

When i use FILE > EXPORT > (=export as a animated gif) with the option 
delete previous frame, then the animated Gif  looks fine. but the size is 5x 
bigger than when converting the first option ( i understand why)

how get rid of the black lines ??

Attachments:
* http://www.gimpusers.com/system/attachments/45/original/error_in_gif.gif

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fvn18 (via www.gimpusers.com/forums)