strange repeating rings on prints

hello everyone,

ever since ive really built my mendel90 ive been getting these weird rings on all my prints, its more noticeable on higher layer heights but all i had to hand was my yoda as i've been unable to print anything properly for over a week due to crappy quality plastic (avoid 3d karma).

as you can see on some of the images theres like rungs every set distance i assumed it was every turn on the lead screw or something along those lines so i did the following:

making sure the lead screws have enough oil
dismantled the z axis gave a full strip clean and rebuilt and did a total recalibration (improved things but didnt resolve)
changed oil

if this is just a common thing and i should just lump it is there anyway of trying to minimise its effect?

any help is well appreciated

also as a side note ive noticed that when my print starts doing the usual movements in nophead initial code, sometimes when the x axis moves the z axis motors rotate slowly with them, i thought this was some sort of interference or crosstalk, not sure if this is anything to do with this but hay ho ill get everything fix at some point.

Have a read of [reprapmagazine.com]
The article you need starts on page 29.

richgain