voltage drop
I have the 2001 and it doesn't surprise me that you missed the voltage changes when the air intake heater is on and the glow plugs cycle. I have only noticed it once or twice since I bought this truck, the first time I noticed it I dam near had a heart attack seeing the needle way down there then I realized what was going on when it bonced back to normal.
Al
Original D.P. Member #750, 2009 Ext Cab LB 4x4 Duramax/Allison, Black, Linex, 1993 6.5 TD 4x4 reg cab LB, 5 speed, 1972 Pontiac Lemans, 94 cady De Ville