I noticed that the X-Particle emitter (if you set it to Pulse emission) is emitting in shorter intervals if you enable motion blur.
I guess this is because the Emitter also reads in the additional sub frames that are needed for MB as actual frames and thus the Interval is not what you get in the Viewport.
It might be a X-Particles bug. But I post it here anyway so you know about it!
Cheers,
Raphael