Laser Engraving - Slowly Losing Steps in X Axis throughout project or "drifts"

Hi all, first time here on the forums. Just some info up front, I am a tech savvy guy, but I know nearly nothing of votages, motors, etc. I have owned laser cutters for about 3 years now and I've assembled my own CNC milling machine from Inventables... but I am not an engineer, just a curious man who likes to build things.

So here we go. This issue has been haunting me for about a year... I have a 100w Redsail X700 (700mm x 500mm bed) with a Ruida controller, running on RDWorks. I've been unable to get any customer support from cncCheap (the ebay seller) who I bought from.

When I do large engrave job, the x axis slowly starts to drift to be offset, as it progressively moves down the Y. You wouldn't be able to tell, until you go to cut it out. The cut is accurate at the bottom of the project (closest to the last engraving) then slowly becomes terribly offset toward the top (furthest from the last engrave). So basically, the x axis is somehow becoming offset from its true positioning. See this image for a better explanation:

http://i66.tinypic.com/2njd9w2.jpg

I've worked through a lot of possibilities and I think it has something to do with a stepper, voltage or software issue... this is also happening to a couple of machines of other people I know, who run various versions of Rdworks. I am pretty certain it is not mechanical, as I have checked all belts, wheels, made sure no movement is happening on the table, and the beam is focused everywhere. I've also ran the program in many different ways, running the engrave and cut as two separate jobs, running it as Udisk, all one layer, all one jpeg, multiple layers, putting it all in a rectangle "frame", etc. Same result. The same thing also happens on other large projects.

I have attached images so you can see what I am talking about.

Top of project (terribly offset, as it is the furthest in the timeline from the engrave job)

http://i66.tinypic.com/aym5pi.jpg

Bottom of project (perfect at the bottom, where engrave just finished and now begins cut job)

http://i66.tinypic.com/2jc91lw.jpg

Does anyone think it could be a voltage problem? I see some potentiometer dials on the drivers that are connected to the x, y, and z stepper motors. Image below:

http://i64.tinypic.com/fz223k.jpg

Maybe I need to just replace a stepper driver or stepper motor? Anyone solve a similar problem to this? Any idea how to fix it?

Thank you for any help you can provide!

Sincerely,
Jon