Added some printed trussing to top of i3 to stiffen framing. Back M3 chord is tensioned up a bit, makes frame way more solid. Went cheap with MDF to start with and suspect movements of frame, particularly on bigger prints, were affecting result quality.
Noticed that was getting slight wavy effect in wall of models, looked like canning you get on metal plate on ships, as went in and out of resonant frequency of something as print height increased. Hoping trussing will address somewhat.
Thanks for sharing, any chance you took some measurements before and after installing the trussing of vibrations or natural resonance frequency, please.
No. Was really ‘by eye’. Can see waves at 21secs in this video on edge http://youtu.be/0uamQGfFPM4 Have since reprinted with exactly same setup but with stiffening in - no waves now. Suspect as weight of Extruder etc goes up the vibration frequency decreases, like lengthening swinging pendulum. Plus any rotations of print bed amplified as print height increased. Just interested in squeezing a little more performance out of something pieced together with MDF and wires ripped out of old hair driers etc. Never ceases to amaze me the thing works and prints stuff! 
I had a similar problem with the frame wobbling. I’m very proud of my design for easy to retrofit stabilisers https://www.youmagine.com/designs/i3-frame-stabilizer-15-35-degrees
@Richard_Mitchell nice and simple
@Richard_Mitchell like it. Tidy. MDF/timber version has large buttresses in same plane as your supports, don’t think alu one does right? Timber frame actually pretty solid in that direction. Great solution for metal prusa though.
@Jon_Robinson yeah the wobble only affects the metal frame i3 and even then only if it is on a table that isn’t solid. I made mine worse by adding the spool on the top.