Graphics-bug in the new minigolf-track

While working on the minigolf-track, i added grass into the water.
Some of these plants in the water doesn't look normal (it only happens for plants in the water)
When i drive along the road (where these plants are), first it looks normal but then it changes.
here you can see an image of that
download/file.php?id=6827&mode=view
and here is a version of the track, where it happens for me
http://platen-software.de/sebastian/min ... -v1.tar.gz
Please test it with this track-version, you should drive this part more than one time if you can't see this bug on the first time
(If it doesn't happen for you, it could be something with my graphics-card)
Have you an idea, why i have this bug?
Some of these plants in the water doesn't look normal (it only happens for plants in the water)
When i drive along the road (where these plants are), first it looks normal but then it changes.
here you can see an image of that
download/file.php?id=6827&mode=view
and here is a version of the track, where it happens for me
http://platen-software.de/sebastian/min ... -v1.tar.gz
Please test it with this track-version, you should drive this part more than one time if you can't see this bug on the first time
(If it doesn't happen for you, it could be something with my graphics-card)
Have you an idea, why i have this bug?