falling true mesh

I'm falling true my mesh after some time.
Any idea what can cause this?

I have a video my scene where the problem occurs (.avi 3mb)

http://www.dwimmo.be/wheelchair.rar

all help welcome!

i'm sooo ***** :p
beginners fault: I did put the setworldsize as comment
now everything works

yesssssssssss

btw still dont get why the treecol didn't work properly when set in a seperate function anyway... I know what the cause was so now I avoid seperate methods...

thx all for helping me!