Here’s another single string study. There are some heavy metal riffs based on this kind of idea (eg: Yngwie Malmsteen), but they require higher tempos. The MIDI files here are all at 85bpm, but you can change that with Anvil. (See blogroll). Start slow and work your way up to as fast as you can go! Malmsteen would manage 140-150 no problem.
MIDI for first variation — playing triplets. This will teach you the position changes. Those changes are the same in all three variations.
Second Variation — 16th notes. At the same tempo, so harder to play because there are four notes per beat rather than three.
Final Variation — a 2 bar phrase is used through the chords. Only a little harder than the second variaiton, but only until you get the phrase under your fingers.