asymmetric NFS transfer rates

Sorry if you can't hear any more performance problems, but I'm wondering why I 
can't write to my 5.3-stable NFS server more that 3,5MB/s while reading gives 
me 9,5MB/s?
Server has a "em" card, client is also 5.3-stable with "fxp", but if I try it 
from my debian box with a "em" card avarage write performance is also bad but 
with strange peaks. The first second almost no data gets written to the disk 
and then it peaks up to 7,5MB/s, but like said, average is also about 3MB/s.
The server can easily handle 35MB/s (twe 4x160GB R5) and was idle while 
measured!

Thanks for any clarification (I tried different -w and -r sizes, but that 
hasn't changed anything) or confirmation of that problem.

-Mano