If you have the pops firing at random, try hitting the playfield softly with your first. If they pop - then regap. If not, the problem is somewhere else in the switch matrix. Best way to fix this is to replace all the caps (as discussed earlier).
If you really want to isolate the faulty cap - replace all the caps on the switches in that row the firing pop is in. If the problem persists, replace all the caps in the column.
I once had a problem on my Space Invaders that ended up being the PIA socket on the MPU board. One pop would fire at random, even with the ball still in the plunger channel !
