Try out this test: Have the PitchFactor in Play mode before engaging the MPC. When the HarPeggiator starts freaking out, press the middle footswitch [RESTART]. Does the extreme rate of the PF settle down to the MIDI Clock BPM that the MPC is sending?
I've had similar situations with an external clock, and the PitchFactor in HarPeggiator mode. As I recall, no one else could reproduce it. (I posted a recipe here. I'd be interested in your take on this.). The only way that I could get the PF to audibly snap away from (an apparent) maximum internal tempo was to hit the [RESTART] button.
I can't cite a particular 'factory' preset to test this on. I experience it most often with custom presets that vary the speed or time division by some other means (MIDI, expression pedal).