working on a 02 300 for a friend. starts up and runs great for the first 5-10 minutes then starts sputtering and running bad throughout the rpms and wont hold an idle. plug is black and sooty like its running rich but i think its an electrical issue. after sitting for a couple minutes its back to running great again. i have put in a manual petcock and confirmed that its not sucking fuel through the fuel pump vac hose. regulator rectifier tests out fine. is there a way to test the cdi? or does this sound like maybe the coil is the issue? i havent checked it yet

 

I assume you have fitted a new pug so, if you are sure it isn't a carb problem ( a sticking choke is the most obvious), substitution may be the only way to go. You could try checking exciter and trigger coils electrically, but they probably won't show the fault until the engine gets hot, if at all.