Speedometer/tachometer and wiring

I am having a problem with my speedometer and tachometer going to zeroing and bouncing around a bit for a few seconds then returning to normal. My mechanic mentioned they saw wires ‘arcing’. What does this mean? What wires would be doing this? How much would this normally cost to fix? He also mentioned a full tune-up. Why would this help? THanks!!!

Well? On a Peterbilt…?


Sorry, on my 93 Honda Civic…