Print head (x-axis) wobble

Recently, when trying to print some large items, I've noticed some pretty bad distortions in the print on surfaces extruded near the back left corner of the print bed.  When I watched the print in progress, I saw that force from the compressed tether cable was actually pushing the print head forward so that it would tilt every time the head moved into that corner.

Has anybody else seen this?  I was able to temporarily fix the problem by taping some heavy washers on top of the cooling fan behind the extruder, but I'm not completely content with this solution.  Others have suggested closing the gap by adjusting the blocks but this seems like a bad idea on its own, as the gap is presumably there to prevent binding.  I'm thinking of designing something spring-loaded to replace the lower Teflon block, but wanted to know if anybody has already done something similar.