I'm guessing here...but
AFAIK the electronics run on 9V, not 12V. I remember hearing that was why a battery test under cranking was so important(if it drops below 9V, the engine may still turn turn but the Igition Module won't work).
-The blowing starter fuse(120+watts consumed) indicates something big shorting in the cranking circuit through the key.
-The intermittent-ness of the problem indicates something moving(starter/relay/wire connection) that connects only part of the time.
(but it still could be electronic)
-The bouncing Tach indicates pulsating voltage backfeeding(bad plug/wires ground to eng/batt) through the guages.
Tried to simplify it. The red arrows could be 12V leakage into another circuit:
My best guess.