D20 Autofokus problems

hi everybody

my new canon 20D (4 days old) with sigma 18-125
produces mostly pics which are NOT SHARP
when using AF one center ( -with multiple center even worse).
with a canon objectiv same problem - maybe a bit better result in general.

but when i focus manually with each of the objectives - the result is brilliant !!
i have seen more people posting the same issue.
any experiences with an exchange of the camera ????
is it a problem of my copy ?? or generally ???
does someone have a suggestion ??
would really really apreciate it
before i start to talk with my dealer for a satisfying solution

thanks for coments
greeetings chris