Matt, this could be an instrumentation failure. The ECM is starting to appear on the horizon. Ground wires can be the culprit, too. Check where batt and ground connects, usually to the block. If so, then you have another cable connecting the block to the firewall (dated info). Maybe your meter set to ohms, connect one lead to alternator housing, other lead to frame. Look for zero ohms, then start moving stuff around. Then get the meter leads out of the way and start it. Watch the meter, it should not waiver.
You're past the easy stuff now buddy. Good luck.
I have a short attention spa