2002 Chevy Silverado 5.3 showing po300 code. Symptoms are - (in cold weather) fires right up then rpm's drop & it runs very rough until it starts to warm up. It runs smooth once its warmed up. It's just the initial start up (in winter weather) when it chugs & runs rough during idle warm ups. The colder the weather the worse the problem is. Throttle position sensor? EGR? O2 sensor?

run the fault codes
check for vacuum leaks on all vacuum hoses
check fuel pressure
use quality injector cleaner

When was the last time a full tune up have been done? A bad intake gasket would do that.

Mine did the same thing...at 42F degrees, run bad. After it warm up, it would be fine. This would only happen in the winter. It would throw a code saying it was lean in Bank 1 or 2.
The issue is a contraction of the intake gasket due to the cold weather, when it contracts, it leaks air. There is a new, improved gasket now from GM, or get a Felpro from major auto stores and it will work fine. I consistently had the problem every winter, and this has fixed the problem for the last 3-4 years.

sounds like coolant is getting into the combustion , i would start with no2 cyl , its a place to start, could also be a leaking injector,

Symptoms are indicative of a partially broken wire somewhere, the cold increases the resistance in the wires and if they aren't working at 100% to start with, will cause your problem. I think you are on the right track, I would replace any high tension parts not already new then start looking for wires showing a high resistance. Check and clean ( with contact cleaner aerosol ) any and all electrical plugs in your ignition system. I would try drying out your computer and packing around it with silica gel to keep it dry.
Good luck

DTC Number
P0160 DTC Name O2 Sensor Circuit No Activity Detected Bank 2 Sensor DTC Number P0300 DTC Name Random/Multiple Cylinder Misfire Detected

check your glow plug relay! located on the right side head

give a good tune up. if it still is acting up let me know

After fighting with mine (2003 GMC 5.3 liter), I believe I have it solved. Mine was throwing P0171 and P0174. Also, there was an occasional p0101. I had a rough idle sometimes right when I started it, but usually after 5 minutes when the fuel system went in to closed loop mode (about 200 RPM swings at idle that slowly went away). Days below 10 degrees were much worse.

I cleaned the MAF sensor, checked fuel pressue, changed spark plugs, and cleaned the throttle body. The code basically means that more fuel is being dumped in by opening the fuel injectors longer. The code is set by the long term trim on the injectors be over 23% (opened 23% longer than they should be based on sensor readings). The extra fuel allows the engine to run with the right air to fuel ratio (not necessarily lean as the code suggests). Causes could be low fuel pressure (from pump, fuel regulator, clogged injectors, and such). Running a scanner on it for several days, everything looked normal.

That is all background. The other cause is that air is being introduced into the intake after the MAF sensor. Extra fuel has to be added for the air that the MAF does not see. I did some digging and found out that the gaskets between the plastic intake manifold and cylinder heads are prone to leaking. I tried to spray throttle body injector around where it might leak to monitor the engine speed, but the air leak seems to be less on warmer days and leaks can occur on the inside edge of the gasket where it is hard to reach with the spray (there are foam ends that open the inside edges to atmosphere). I replaced the gaskets and my long term trim dropped to about 6% from 10% on warmer days to pegged out at 25% on cold days with a cold engine. The work isn't too bad as long as you are patient enough to get the plugs off of the ignitors. I have yet to test it out in cold weather, but I am pretty confident.