Sean. Sounds like a high resistance in the ign fed positive side if you can make it go away by powering from another source. Either (and would have thought others would have the same problem) the cable is not man enough that feeds the ign circuits of the fuse box. OR Are you still using the original Sierra Ign Switch? If so the Ford connector on the short loom of switch wires are renowned for the connectors breaking down! Worth a wiggle
