Help with printing on Vertex 8400

Can anybody give me some advice.
If I print on my Vertex 8400 the item that I print
has a lot of horizontal lines visible.
I thought it might be the Z-axis and
for that reason I printed a Z-axis stabilizer fitted
with a 608 bearing. Unfortunately the results stay the same.
A little play on the X- and Y- Shafts, mite that be the reason?
Thanks
Paul Roos
South Africa