I have been playing with it today, i had one screw in each bearing block tightened up, one side of the frame loosened off so it would move but it is still binding and hard to move, i even tried adjusting the bearings but no joy.
Sorry I meant the ballscrew/leadscrew

I took the y axis frame off leaving the 2 plates on, and if u put any slight sidways pressure on the plate it locks the bearings up.
With a leadscrew on each side and linked with a belt, (which you will need anyway with the ball bearing rails) then both sides would be moved evenly and should'nt bind.

but then again if you can get the bearing type then that will solve everything. I've got 20mm THK's and they are solid..

If you are still going to use the igus on the y axis then try to follow the design guide on the width to truck spacing ratio, also make sure the leadscrew is perfectly in the middle otherwise you will have the same problem with binding....especially with the 250mm z axis.

I would try again with the igus on the x axis with twin lead screws and use the bearing type on the y axis. just my 2 penneth

out of interest what are you using for the spindle?