Code P0300?

It means random/mutiple cylinder misfire detected. Sounds like your in need of a tune up. When was the last time the plugs ,wires air filter etc where changed. If it has been awhile maybe it is time to do a tune up. Then see if the code comes back.
BYM :?
PS Next time please included year, make, model and mileage of car. It will make things easier.

changed all that,new plugs,wires,distributor cap,rotor ignition control module,fuel pump! 2000 GMC Seirra 4.3

If you ran a scanner and did not pull any other codes then it is going to make it tougher to find.Have you checked and tested the ignition coil. If so it and it's good then I would suspect a lean fuel condition. The key here is to figure out what is causing this. A vacuum leak in the intake manifold a EGR value stuck open or not fully closing. Just to name a few. Hope this helps BYM :)

WHEN DOES IT DO IT? IN AM WHEN VEH. COLD OR HOT OR DOES IT MATTER . THEY DO HAVE A PROBLEM WITH VEH. POPPET VALVES STICKING, NORMALLY COLD BUT SOME TIMES THEY WILL REMAIN STUCK REGARLES, U WOULD NEED TO FINF OUT WHAT CYLINDER IT & DO IN ENJECTOR BALANCE TEST THEY NORMALLY DROP ABOUT 7PSI PER INJECTOR SO IF U FINF ONE THAT DOESN'T U HAVE A POPPET VALVE STUCK ... GOOD LUCK