Head rises when moving towards the centre? (4 replies)

Hi all.
I'm making a Delta printer, based on Rostock Mini design.
Basically, it works fine, and i've calibrated it to move flat and to scale.

However, there is a slight problem.
When the carriage moves away from the centre, it moves flat.
When it moves towards the centre, it tends to rise a little, especially closer to the edge of the build area.
If you order the head manually 10mm, 10mm forward, then 10mm, 10mm back, this is what you would get:


I.e. if you print a thin rectangle from centre, then the side will be flat to the edge, flat acrosswise, then rise and fall in a curve as the head moves back towards centre, then flat again.
We are only talking 0.2-0.3 mm rise at worst, but it wrecks chaos on larger builds with fine details on first layer, and might result in a nozzle crash with resulting offsets ruining the build.

What could be causing these rises?
It feels suspiciously like some sort of backlash, but i'm using GT2 belts with machined pulleys, and tightened to a note, so it's not obvious where to look.