i dont know if anyone has had this come up.
but iv been dealing with crazy egt's for a few months and a boost leak.
i think i found my boost leak. one of the boots off the spider has a pin hole that i think is opening up under pressure.
but today when i started my truck i notaced that the glow plug mod led light was on, and my egt at idle was 300* thats ok,
but when i put my foot on the breaks my egt needle jumped up 50*
i did this several times to conferm that it was the breaking action doing it.
as soon as the glowplugs went off this went away and back to normal.
so i killed the engine and re started same thing but when the breaks pumped up this went away.
i looked under the dash and cant see any guage wires around the break pedel, that might have been rubbing.
kind of crazy.

 

Maybe weak batteries, corroded connections? Try it with turning the headlights on and off too, worth a shot. Voltage drop due to brake lights being actuated would vary the voltage for the gauges and change values.