Originally Posted by dhjenkins

You should have let the tank empty out before adding the fresh gas, epecially if you aren't buying quality gas (toptiergas.com).

My MB had about 8 gallons of 'bad' gas in it (from sitting) which I put in my '00 Sierra. Truck ran like crap, hard start, stumble, etc... basically the driveability you're describing and only got slightly better when I added 10 gallons of premium (only requires 87).

This time I ran the tank almost dry before refilling, and full tank of 87 had it back to normal.

Still, it wouldn't be a bad idea to check and see if your pumps are running, how they sound & what kind of current they're drawing.

If you haven't changed the fuel filter(s) lately, that might be something else to look at.