AP Roaming with RADIUS Auth WLAN


i´ve a WLAN here with RADIUS (Certificate based) Authentication set here. I can connect to the WLAN without Problems ... RADIUS Auth works ... but when im "travelling" through my building as soon as there is some Roamin from one AP to another the Network Connection drops immediately (sometimes it reconnects after a few seconds ... sometimes not).

With WPA-PSK the Connection is stable throughout the way ...

Any Ideas whats possibly wrong here ?

Thanks and Regards