No buffer space available

Hi,

I'm having trouble with qos and openbsd3.3, basically I do ACKs 
prioritizing (www.benzedrine.cx/ackpri.html) and when there is a lot of 
outbound traffic (128kbps is the max on my adsl link), the network 
traffic stops after a couple of minutes, except already established 
connections. If I do a ping I get this message "No buffer space available".

I've stfa and there seem to be other people with same problem, so as 
suggested in other posts I've tried different NICs and increased network 
buffers via sysctl but it does not make any difference.

When the problem occurs, if I reload the pf rules with pfctl -f 
/etc/pf.conf if works again for a couple of minutes. If I remove qos 
from the rules, there is no problem, so it has definetely something to 
do with qos!

Any ideas ?

Regards,
Alexandre