WiFi handoff problem

Ok, I've got 3 PicoStation2 access points, currently flashed to DD-WRT software. Two are connected to the LAN, the third is connected via Client Bridge to one of the others.
They're on separate channels, same SSID and same WEP key.

If I roam from the first AP to the back AP, the wireless part seems to hand off fine. Netstumbler shows that I switch from one MAC/Channel to the other MAC/Channel. However, running a constant ping back to the firewall, as soon as the switch occurs I drop packets for several minutes. As long as I don't physically walk from one to the other both AP's work just fine.

Fortigate 60B Firewall is generating DHCP, connects to an HP Procurve 2510. The HP does have spanning tree enabled but it probably doesn't have to be currently. Fiber runs out to the far end and are connected to secondary small switches.

Any ideas?