You probably just need to move your distributor a tooth or two. A timing light is for the fine tune, but the mechanical relationship of the crank, to the cam to the distributor has to be right to start with.

Set up your engine as if you were putting on a new timing belt; #1 cylinder top dead center, timing mark on the lower pulley "0", timing mark on the top gear with the dot matched up with the notch. Pop off your distributor cap and find the contact marked #1 on the inside of the cap. Make a mark on the outside edge of the cap that corresponds to the #1 contact.

Put the cap back on and make a mark on the outside of the distributor body that matches the position of the mark you made on the cap. Now pull the cap off and see if the rotor is centered on that mark. If it's not,
remove the bolt that holds the distributor in place and gently pull it straight out (let the rotor turn). Not all the way. Just enough so the gear on the end of the distributor shaft clears the gear on the end of the cam shaft. Then turn the rotor either clockwise or counter clockwise (depending on which way it was off) hold the rotor steady in the new position and slowly push it back in in until the gears mesh again. Release the rotor. Check the position of the rotor to the marks when you've pushed it all the way in. It might take a few tries to get it right, but take the time to do it. The valves don't like to clatter and will reward you with a time and money consuming rebuild of the head if you ignore their chatter.

If the harmonic balancer has slipped you will not be able to line up the timing marks on the harmonic balancer and the timing marks on the cam shaft timing belt gear in the #1 position, but that's assuming that it was assembled with the two of them lined up in the first place.

Of course if you do all the above and it all lines up and nothing else is wrong you need to change gas stations. All gasoline is not equal. The vac lines have some influence but not enough to cause excess ping.
The only two that need to be plugged for timing are the EGR valve and the vapor recovery.