Platform moves VERTICALLY and horizontally during one layer

[matte]

When printing a wide object, after a few layers, the platform moves down impacting the bed a few times, then moving vertically. It continues the print, but for all the next layers, the platform moves vertically +/- about 10-15mm (for the same layer) as well as moving horizontally.

1. this is the first object I have printed that is really wide/covers most of the bed surface. I have printed vertical objects and smaller objects successfully. Its at the point where it should just work and I wouldn't expect a problem like this to happen.
2. I tried setting Z=0 at 20mm above the bed, and this problem still happens. The nozzle doesn't impact the bed, but continues moving +/- 10mm during each layer, in addition to horizontal movement. This indicates its not impacting/bouncing off the bed that causes the vertical movement during the same layer.
3. I have checked the platform is level and that the bed is level and the nozzle is level.
4. I'm using the stock platform and stock Configuration.h with injection modeled carriages
5. The Horizontal Radius is 141mm, which travels level over the bed from all 3 towers during calibration
6. I have remeasured the arms, horizontal offset, platform radius against what's in the Configuration.h for Rostock Repetier 0.91 and they're the same measurements (as accurate as I can tell)
7. I tried replaying only the G-Code of the problem layer, but only to find that that layer(7) doesn't always cause this problem, sometimes its a later layer(23).
8. Reading the G-Code for those layers, no code stands out
9. I am trying to print the STL file from http://www.thingiverse.com/thing:364325" onclick="window.open(this.href);return false;

Googling hasn't found me the answer to this problem.

Let me know any info that could help.

Please let me know anything more I could try.

[IMBoring25]

I would guess something is binding in the motion of the arms at one particular X/Y position and causing missed steps, and it's all over from there. On a delta, the machine had to know where the effector is accurately to even move in the right directions.

[matte]

Where could that be? Where the arms join to the carriages?

Whatever the problem is, I think it only occurs once and then its all over, so sure could be caused only at a certain position.

I have tried slowing down and speeding up the printer and the problem still happens (just in more or less time, maybe a slightly different layer).