Midi oveflow?

[mantik]

Hello,
problem [not solved]:

cubase sx3 is connected with a midikeyboard and virus classic, with an
midi-interface (midisport 2x2).

here is the setting:
keyboard[MIDI-OUT]---------[MIDI-IN]midinterface/cubase[MIDI-OUT]-----------[MIDI-IN]virus

when i connect in the miditrack of cubase
IN:Keyboard and OUT:virus
so that i can rout the midinotes from my keyboard into the virus, the virus hangs up sometimes(endless release of the note), sometimes cubase hangs up(frozen). the crashes happen sometimes, sometimes not.
playing the virus with sequencer is no problem, also no faults caused by connecting the keyboard from midiout to virus midi-in directly.
but if i connect both with the interface.... > crashes



please help!!