/diy/ - Do It Yourself

Hi everyone,
I have quality issues with my printer and I can't really find the source. Does anyone of you have an idea?
One issue is, that I changed a lot of things at once, I tried to change parts back, but I just can't find the issue.

firmware: Skynet3D
slicer: simplify3D
printer: Anet A8 with a bunch of customazations
customazation:
- anti z-wobble coupling
- motor damper
- 300x200 bed
- 24V for the headbed from 2 ATX power supplies
- 80mm instead of 60mm rods for y-axis and used the 60mm rods for the belt tensioner
- the printer is screwed down onto a concret plate
- bowden system with the extruder attached to a shelf
- E3D V6 clone with PTFE and all metal (tried both) hotend
- 2 mosfets so the current to heat the bed/hotend doesn't fry the board
- ...

My issue:
Like you can see in the picture, I have periodic underextrusion.
First I thought it might be z-wobble, but after looking at mutliple prints, I'm pretty sure that the head movement is fine, but there is just to little plastic coming out.
The pattern is very consistent, but sometimes it shifts and sometimes it doubles in frequence.

What I have tried so far:
- I printed without a headed bed (I thought that there might be a voltage spike everytime the heater draws current)
- I used 2 different power supplies for the hotend heating and the steppers (main board)
- I tried it with a different extrusion motor
- I tried out different anti-z-wobble methods (I'm very sure it has nothing to do with that)
- I printed a pyramid to see if the paddern frequency changes. It seems as the gap between under extrusion gets smaller but then at the top much bigger.
- I printed an object in vase-mode, to make sure it has nothing to do with retraction.

What kind of helped (at least a little):
- I used a new nozzle and put thermal paste between hotend and cooler
- I put a little more tension on the extruder spring, so the filamet gets gripped a little tighter