MPC Master - Nuendo Slave (tempo problems)

Using MPC2000XL to control Nuendo, everything works fine, except when I try to toggle through the bars with the data wheel, it goes totally out of sync. On the MPC it says bar 20 (for example) and in Nuendo its only shifted a few bars. However, during normal playback everything is perfectly in sync. What could be happening?

Nuendo can't be used as a MIDI slave.