Skipped layers

[Reaping miner]

Okay, i think i solved the problem. I woke up in the middle of the night for the usual reason people generally wake up. And i had an "eureka moment".

Hypothesis:
I hypothesized that i tightened the X-axis belt too much. causing the X-axis ends to be closer pressed to one another as there is no stop or end in the ends that prevents the smooth rods to slide too far in. This caused extra friction on the Z-axis linear bearings. In turn, this causes the Z-axis motors to convert rotational energy to compression energy in the flexible helical Z-axis motor couplers (basically a loading a spring). This causes a lag in movement after the Z-axis direction changes as the couplers release their energy stores first, until the drag force of the coupler overcomes the resistance of the Z-axis bearings.

Effect:
After homing: Overextrusion at print start
For homing the Zaxis moves down, then up a bit. and then again down, extending the coupler. This causes the coupler to compress for the first few layers, instead of increasing the Z-height until the coupler reaches an amount of compression that compensates for the Z-axis bearing binding.

Normal printing until the Z-axis direction changes: (i.e. the first Z-hop. (Before that exclusively upwards Zaxis movement.))
The Z-axis moves up the normal amount for the Z-hop, the motor couplers are already compressed.
However, when the Z-axis is supposed to move down, the motor coupler first releases its compression energy before dragging the Z-axis down after it is stretched sufficiently. Resulting in less downward movement than expected and printing too high.
This problem slowly resolves itself as the motor coupler gets more compressed during subsequent, non-z-hop layers (The Zaxis does not move, or moves less than it is supposed to, during this period) and layer height gets back to the set value. (Subsequent consecutive Z-hops would simply mostly compress and decompress the couplers and not actually move the Z-axis much during the Z-hops. However, the resulting net upward movement of the Z-axis will slowly compress the couplers on normal layers, much like at the start of a print.)

And then this cycle repeats on the next "first" Z-hop.

I printed my diagnostic cube with nuttrap with only minimal artefacts at the first Z-hop layer of the holes/nuttraps.

The solution(s):
Less tension on the X-axis belt (duh.) -> helps dramatically
Z-axis pretensioner -> http://www.thingiverse.com/thing:1044542 (Ironically i had these parts laying around for about a month now, but no bearings to install it. So i ordered these, among other things, yesterday and they should arrive today. So will also install these and let you guys know.

EDIT: Yes the pretensioner also helps, it does introduce something that looks like a small bit like Z-wobble, but small enough to live with. I will post a picture of the newly printed extruder body later. (If i do not forget.)

Strangely enough i havent been able to find anyone else documenting this problem.