HELP! Randomly no spark!

Carry a alligator clip jumper, BIG, like maybe no 14 wire and "old school" full size clips. When it quits jumper from battery direct to coil and see if it will start. If it will not start using the key, try cranking it by jumpering across the starter relay. If that does not work, also carry a couple of jumpers made up so you can bypass the resistor, or maybe even just go ahead and buy one and try it.

Along the way, loosen and clean the mounting bolts on the ECU. The ECU MUST be grounded. To find out, turn key to "run" and measure voltage on both sides of coil. PLUS side should be somewhere around 6-8V not much higher. If it is "same as battery" IE around 12, the coil is not drawing current. Measure voltage on coil NEG. If it is 12V the coil is not drawing current through the box. If it is 1-2V or so "normal." If it is zero the coil is open assuming you have power on the PLUS side