Have you ever seen a traffic shape like this ?

Here is the description of my problem:

-a 2950 acting as Aggregation switch for a few access switches ( most of

them are Cat2924).
-the Aggregation Switch has an 1Gb uplink to a Cat6509 (truncking,
dot1q,
all vlan allowed on this trunk)
-L3 switching is setup on the Cat6509.
-some other swithces 2924/2950 connected to the Cat6509.

See the diagram here:
http://www.geocities.com/cristi_piatnitchi/index.html


Any client connected through the Aggregation switch has problems with
uploading files on the network (whether is on a server or an other
client
workstation not connected ). The downloading works fine on those
workstations.

Any transfer between workstations (upload or download) connected through
the
aggregation switch works fine!!!

Everything else looks fine to me: no error on ports (whether they are
uplink
or access ports), the trunking settings look fine.
What else could be ?
The strange thing is that the traffic is chopped like the digital signal
(snmp should show me a solid line, please see the link)

Thank you in advance.
Cristian