2003 Hummer h2 p0300 code
 

I have recently been getting a check engine indicator on and found it was a P0300 code multi cyclinder random misfire, I have replaced the plugs, wires and all coils. It only happens on a cold start. When the vehicle warms up after I have cleared the code it does not come back until the next day when first started up.

Might have better luck finding more out about the engine code and what might be causing it on any of the many GM Truck forums. The GM Silverado and Sierra HD trucks as well as many of the SUV's use the exact same engine and are much more common so the likelihood of finding many that have had the same codes is much higher.

Any backfiring?
May be the usual valve seat issue, if memory serves well P0300 is that, but not sure, so look into it.

Welcome to HF. I see you figured out tech question do not go in this section, but should be in the vehicle sections.

Good luck.