I'm having this issue too. I haven't upgraded the firmware yet, but I have tried turning the tempo down to a minimum, as suggested above; no luck.
The arp drifts out of step; it's noticeable after maybe 10 seconds. At first, I thought I wasn't tapping the tempo properly, or that the button wasn't responding, but it starts in sync, then moves out of step very quickly. I don't have any MIDI devices connected to the unit.
Note that in the PC editor, I have set the option to repeat the notes in the order I played them (I mention that in case that might affect things).