Difficult P0300 misfire trying to learn
 

My 02 a6 2.7t quattro 6speed manual has about 164,000 miles
It has a small hiccup at idle but not when I give it a little gas
It has a hard miss at about 3500 rpm but only when under load
I've replaced the spark plugs, ignition coils (boot and all), intake gaskets, fuel filter and ran seafoam, and inspected my pcv and vacuum hoses.
I don't want to take it to a shop not because I can't pay for it but because I want to learn. I've spent hours reaching all sorts of problems non specific to me but have found no answer for my problem

codes? maybe bad injector(s)

The only code is a useless P0300. They looked good when I replaced my intake gaskets, how would I test them?
I was wondering if it was a faulty combination valve or something