I'm not going to get into all the timing issues others have tried to cover and just tell of an incident I had happen.
Can gas go bad sitting three weeks in your car?
Maybe. The key is, how old was it when you put it in?
My wagon has a stock '77 440 in it. Compression ratio is in the 8's. Back when I was daily driving it, I was running mid-grade or higher because I kept getting slight pinging under load or at WOT.
I tried everything tuning/carb related to get rid of it, to no avail. It was especially frustrating because I knew the comp ratio was so low.
Then, one day I needed to get gas, but found myself somewhere that had only the 87 swill that everyone uses. I expected it to ping real bad after all the issues with the higher octanes, so, imagine my surprise when there was NO pinging.
No matter how I ragged on it, nothing.
All I could figure was that no matter the station, the higher octanes don't get used as much, so, they end up sitting much longer than the cheaper swill and basically going bad before they even hit your tank.
From that point forward, I used nothing but 87 and never had an issue.
Now, I'm not saying go run out and fill your tank with 87, but, the gas you're using may be old already and you don't realize it. Unless you have a known source, its a crap shoot what you're gonna get.
My (long winded)