I cant find a spot to bypass the firewall for an 8 gauge cable

fuse

Well it could be the gauge on the dash or the sending unit on the engine. But it could also be low on coolant or a problem with the thermostat. I would check the circuit for the gauge first if the radiator is full of coolant.

http://www.ehow.com/how_4520007_replace-turn-signal-relay-nissan.html
Try this

Sound like you are saying the RPM gauge dropped then a mile later the engine stalled and was able to restart you can be looking crank sensor beggining to fail and it usually fails on the freeway without warning

Look @ the top of your transmission maybe the back, follow the cable that runs from your transmission to the firewall where ur steering wheel would be.

did you tell the parts person it was for a gauge and not a light? Can you trace the wire that attaches to sensor to confirm it is not broken or grounding somewhere else.

ecm is located on r/h side of the firewall. you will see this black plastic piece sticking out of the firewall. its attached with 2 10mm socket bolts. remove those and pull on the box and the em will come out of the firewall.

othere than beingin a tight spot not real difficult just unplug the lead going to it, then remove from the rear of the engin block that is tword the fire wall on the drivers side. you will need a 21mm wrench or a deep well socket

Most of the time there is an inlet screen/filter in fuel tank attached to fuel pump. But there should be another filter between tank and fuel rail. it should be located on the firewall, so if you work backward from an injector toward the gas tank you will bump right in to the filter along the way.

Sound like the computer, is bad sorry to say.