GMC C5500 Duramax Diesel HELP HELP!

Only goes 20mph or so intermitently. Meters/gauges go in and out. speedo,fuel, oil pressure etc.
mileage is 112626
has 4 emisions codes:
p0673 glow plug heater Cyl. 3
p0650 MIL Control Circuit Malfunction
p0381 Glow Plug heater indicator circuit malfunction
p0540 Intake air heater A Circuit

Not sure where to start. I have Motorcycle and Small engine background.
Some automotive of my own stuff.
any help or nudge in the right direction would be great.
New to the whole diesel Game