bad ping-quality of 3com wireless bridge

servas!

i've bought 2 3Com-devices to setup my wlan-network in the house/garden:

OfficeConnect Wireless 108Mbps Router
Wireless 7760 Access Point

the router is connected to the internet and to 2-3 clients via ethernet
and with my laptop via wlan.
the access point ist connected to a media- and linux-application-server

i now have the problem, that:
a) if there is no traffic, i get good ping rates (laptop->router about
1.7ms, laptop->access point about 2.5ms)
b) if i have traffic, the ping rate get's very high (laptop->access
point abount 200-2000ms !!!!) traffic is either streaming of videos or
mp3, even if i start firefox via X-session and surf the internet the
ping rate increases significantly!

i tried to
a) use the access point as main wlan-station with 2 SSIDs and to connect
the router via WDS to one of these SSIDs
b) use the router as main wlan-station and connect the access point as
wireless bridge

any suggestions??????

ciao
H.