I finally got some decent results out of my machine!

Here's what I think helped:
- I reassembled the machine paying close attention to things like the parallel aluminum extrusions being equidistant from one another at the top and bottom and making sure everything important was square and true
- I printed and installed the 2 nut spring version of these http://www.thingiverse.com/thing:795954
- I finally ordered and installed real couplers https://www.amazon.com/gp/product/B010MZ8SQU
- I installed the pulleys longjohn119 recommended https://www.amazon.com/gp/product/B0188IFK4Q
- I calibrated e-steps, set the filament diameter to an average of several measurements, and tuned the extrusion multiplier with this procedure https://craftunique.com/forums/view-thread/694/#7130

No need for expensive upgrades like acme lead screws.