;) Just noticed that you'd said already it's your picking hand... must be going blind.

Tremolo pick to a metronome - 4 notes per beat, 3 notes, 6 notes - you get the idea. Start slow and build up when you can do it perfectly at a tempo. Find out where abouts it starts to break down, and back off 5bpm - and then practice that speed for a while before trying again at 'break up point'.

Bring your other hand in, and start working on co-ordinating the two...

You're right I'm afraid - there is no substitute for practice.