Your USB midiman is probably the problem. It is known that midi through USB has a bad timing (no SMPTE for example). I think that is the problem, but I'm not sure.
Do you have a soundcard with midiconnector? Try that one with your keyboard. If it does play correctly in performance mode, then you can be sure it's the USB midiman.
Maybe you should look voor a external mididevice which is connected through the parallel port (printerport), like the Egosys Miditerminal: http://egosys.net/eng/MT4140.html

Good luck.