That’s interesting. I might have a related issue. I can play eighth notes up to a certain speed, but something different is going on when I play fast tremolo. It’s like a change of gears. I am comfortable playing faster than certain speeds and slower than certain speeds, but playing at certain high speeds is difficult. I am not sure where that break happens, but I’d guess it’s around 220 BPM eighth notes. I can play considerably faster with tremolo (and on a good day it is smooth and even) but I don’t have the kind of control over each note where I could cross strings accurately. I am sure I am not the only person to think, “Man I wish I could accurately pick out individual notes and cross strings at fast tremolo speed.” If I could do that then I’d be pretty decent.
I said it’s like a change of gears, but maybe it’s my brain disconnecting and letting my hand do it’s own thing. Maybe it’s some kind of a mental thing and is self-fulfilling. I know I have done that in many things in life… I convince myself I can’t do something (when I actually could).
Of course, it all depends on what you are calling a medium speed. If you are a blazer that plays 300 BPM+, then your medium speed would probably fall in the range of what I am calling high speed, so maybe it’s the same issue, and you can just control your fast speed better than I. Are you talking about 160 BPM eighths or what approximate range?
I don’t have any sure-fire tips, but here’s my thoughts on a few things to try:
Try playing something very familiar slower than the pace that gives you problems and focus on using your fingers to drive the pick through the strings (more thumb and finger motion). Play slow enough that you can mentally keep control of your right hand. The combination of working on using my fingers more, as well as playing through the strings (like a rest stroke) has led to some really good days where speed comes easy. Once you have played the song for a very long time (30 minutes or more) at the consistent slow pace, try it at a speed that gives you problems.
Maybe shoot some video from several angles of your right hand picking at the slow speed that works for you, the fast speed that works and the medium speeds that do not. If you can use something that does 60 frames per second (like a HeroCam), it might be more useful. Maybe you will see something mechanical going on.
Instead of approaching the problematic medium speeds from below (which you said does not work), try coming at the medium speeds from above. Just to pick some numbers if you can play it fine at 120 BPM or 220 BPM, but can’t play at 160 BPM, try coming down to it from 220 in 3 to 5 BPM increments.
Best of luck and I will say this… I think I’d rather have your problem where you can play fast, but not medium as opposed to the opposite. Easy top end speed is not that common.