Replace Universal Joints and Rods

Well it was time to make another mod to the Rostock 3D Printer.
I purchased some preassembled effector rods with Heim joints to replace the rods and joints I printed originally.
These rods are steel M3 rods with Heim joints (a kind of ball joint) fitted to the ends.

Heim joints are a loop at the end that rotate around a ball.
I have included a video

I still seem to have the printing issue though which basically slightly distorts the object as you move towards the front right corner in x/y.  Its only slight and you dont notice it on most smaller objects, but it is really annoying me. If I print a 150mm square. It is not truly square. It is slightly smaller in the front right corner by about 1.5mm.  Its like that corner is pushed in towards the centre slightly.
It must be caused by the position of the 3 axis as replacing all of the rods did not change the problem. Next step I will make a new base and top to ensure that all 3 axis are perfectly positioned and square.