Missing steps in X axis (firmware)

I've managed to print some test cubes and small extruder gears which print out fine with very little distortion: gears have some flats on the round bases. But when I try to print larger objects, such as the large extruder gear or extruder housing the machine loses steps in the X axis. Whats strange is a tried a 50 x 50 x 5 cuboid to see if it was an issue with the physical size of the piece, but did not encounter any stepping in the print- it seems to be related to the complexity of the print rather than the size.
I've looked through threads about step loss, but what I am encountering is on a radically different scale, the printhead will lose position in the X axis but 4-5mm each time step loss occurs and sometimes returns to its correct position on subsequent layers. I switched the pins for X and Y in the firmware, and the problem presents in the other axis, so it does not seem to be a mechanical problem. I've tried slicing my parts outside of repetier as I also enounter a "could not find the generated G-code file" when slicing complex objects or groups of objects in the repetier host, and while I was able to slice larger and more complex objects, after loading the G code into repetier host, the problem persisted.
I'm using ramps 1.4 with the latest .83 repetier firmware and V0.90 host.
Is this something SD support might fix?

Edited 2 time(s). Last edit at 12/10/2013 06:28PM by blabberjack.

I've added some images of the finished pieces to illustrate the issue, you can see that the two small gears and the 50 x 50 x 5 cuboid printed well but the larger gear is sevrely distorted, steppering about .6 mm. Since my last post I pulled out the rods and libreally lubricated the lm8uu bearings with a ptfe bike chain lubricant, the axis now runs quieter but without any perceptible increase or decrease in printing quality.
I currently have the pololu driving the axis set to .8v, though I have tuned it as low as .4v and it still able to smoothly move the axis at 200mm/s
My settings in Slic3r have speed constrained to 60mm/s for infill and 30mm/s for perimeters, when I turned thee values down print quality was reduced slightly but when turned up very high 150mm/s infill 60mm/s perimeters the steping became incredibly pronounced with the first layer losing about 70mm in total, drawing a spiral rather than a circle when attempting to draw the jagged perimeter of the gear.
Any ideas?

Looks more like your belts are slipping or the steppers are not tuned correctly to me.

I had similar issues when using cheap eBay stepper motors, replaced with good quality and the problem was resolved

This won't happen on smaller items as there is less distance to travel

i have also some troubles with repetier firmware...on 8 microsteps everything is fine but on 16 microsteps it misses steps.
interesting is that on marlin everything is ok. all parameters are same.

but i have thosiba tb6560 stepper drivers

Edited 1 time(s). Last edit at 12/13/2013 02:14AM by woo.

I tried reduing the stepping to 1/8 buit this produced even more extreme work with step loss within the first layerI do have a toshiba stepper driver I could try to hook up but I am hesitant to do so as it's more lilely I'll fry the board than fix it.
My stepper motors are from zapp automations and, as I said when I alter the firmware so that x and y are flipped, the stepping occurs in the new "y" axis, 90 degrees to the position it occured in before being flipped.
I'll try with sprinter/pronterface, though for some reason I can't get my heated bed to work even though it runs fine in repetier.

Blabberjack,

I had the same problem, sometimes on the Y-axis and then again on the x-axis.
I have cleaned up the wire-mesh and shorted the wired to the right sizes. Also put them nice along the frame and now a have no problem with Repetier 0.83 and Host 0.90c on Win7.

That seems to have cleared it up! I rewired all the steppers with grounded shield braid protected cable and routed the wires for maximum distance between cables, not only is the machine quieter but -for now at least- it is producing prints without skipping steps (and it looks really badass with those fat braids)!
Merry Christmas to me : )
thanks a bunch hennyr!