Vertical Lines in Model

Hey Guys;
I'm finally getting back to the shop and doing some work but have came across a very weird problem. I'm cutting an inlet for a horse theme paradise box and am getting shaded vertical lines in my carving. They appear to be slight increase in Z height almost like oscillations. Has anyone experienced this before. I was able to decrease the problem by lowering my acceleration but it is still there. I've tried carving from different angles including 0 (horizontal), 90 (vertical) and even a 45° angle and the lines are still showing up at the apparent same frequency. I'm wondering if my z axis Gecko 320 may be chattering or oscillating. The specifics are: Mach 3 control, Gecko 320's, Chinese balls and screws on a very sturdy aluminum frame. I'm generating the code with Aspire 4.0.
Any suggestions or help will be greatly appreciated.
Bill

I answered this somewhere else that you posted it, but I'm pretty sure its due to vibration or resonance in your machine.

Hey Gerry;
Yup thanks for the answer. The resonance you identified is most likely the problem but I'm fishing for an idea of what would cause it. I'm thinking the Gecko on the Z axis but again, I'm fishing. It's an old 320 thus is somewhat dated but I was kinda hoping someone has seen this before and maybe found a way to adjust it out.
It may be wishful thinking but I don't want to switch out the control system and still have the problem. Also, I'm thinking that if the issue is structural, it would appear on the peaks of the models and not just the flat surfaces. It also could be the g-code math but since it is Aspire, I doubt it.
Regardless, many thanks for the response.
Bill