2008 Grand Prix base model random misfire multiple cylinders. (P300)

Hello,
I have a 2008 Grand Prix base model with 170,000+ miles on it. I have been
experiencing random misfire on multiple cylinders for a bit. It idles rough, rpm
at start is between 500 & 1250. I have a basic code reader and have mainly
seen the multiple cylinders code come up, but I have seen a single cylinder
throw the code during idle, cylinder 4. I have replaced the spark plugs, wires,
coil packs, and bank 1 senor 1 o2 sensor. This is a sometimes issue because
the engine will hit on all 6 and run fine for a month or so, then back to random  
misfires. I think it might be a fuel issue because it seems to misfire less with
higher grades of gasoline. I’m looking for either a quick fix, or a detailed list of
things that can checked with relative ease since I don’t have handy access to
a garage.
Thank you,
DannyIcefire