Have you had the timing checked by a tech (or do you have the right equipment?) i.e. used a strobe at 3000rpm?
The timing can be adjusted by twisting the distributor - readjust it if necessary and make sure it is done up tight. Alternatively if you are cheap and don't want the car set up properly turn it yourself and do no measurements, .. I believe you want anti-clockwise (get confirmation) - just a few degrees should be enough, but turn it more if it's not.
Does it ping when the engine is cold?
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black