Sounds like you have a simple misfire. Chevrolets of that era will not provide you with a code for the specific cylinder that is misfiring. They just throw a P0300–Random Cylinder Misfire. You need to get out of the code menu of your scanner and get to the live data that shows misfires per cylinder in real time and in history. A generic DIY code reader may not have that ability.
If I were to guess, at that mileage I’d suspect an ignition coil failing. But that’s just a shot in the dark.