Peterbilt Voltage Spikes

I have a 2007 Peterbilt 386 that keeps having voltage spikes. I replaced the alternator a few months ago for an unrelated issue. Truck was on the road for a while, no problems. One very rainy night the driver loses all of his external lights (except for the trailer lights) and his dash lights cut out, too. His 4 ways wouldn't even work and his voltmeter was pegged in the red. He shut it off, restarted it, and the voltage was fine. His lights came back one by one. He drove it 300 miles back to the yard, no further issues. I replaced the alternator anyway, thinking bad regulator. I checked his headlight harnesses and wiring, found nothing. Put a new alternator on, been working fine, no issues. Tonight (a very snowy night) the truck just did the exact same thing. I am stumped....

 

Thanks pushbroom I will give it a shot. I checked grounds and what not the last time, but it's worth another look!

 

We ended up running an extra ground from the alternator to the frame and never had another problem. I hope that helps.