maybe a stuck switch.

sounds just like the symptoms of a failed capacitor wired across a playfield switch.

When game is acting up immediately push the coin door test button three times to get to solenoid test. Let solenoid test raise the drop targets and put the ball in the shooter lane (to open all switches). Push the test button two more times for switch test. See what switch number is being reported as closed. Use manual to find what switch that is. Make sure switch gap is fine and its not stuck closed. Otherwise, snip loose one end of the capacitor wired across that playfield switch and see if the problem cleared. If so replace that switch capacitor 0.047uF (473 cap code) 50v+ ceramic or film capacitor. If one capacitor failed consider replacing all of them.

Capacitors increase the sensitivity of the switch for scoring but are not needed. The switch will just be a little less sensitive to quick ball strikes. Most operators cut the caps off by the dozens when they stated giving trouble.

I have the same problem on switch #18 (P target) since many years, now. Indeed, since I have put capacitors on all the brief switches. All of them work, but the #18 : it gives a lot of points when you begin a new play. I have tried several capacitors, to change the target, to change the sense of wiring... Nothing works, except cutting the capacitor. But of course, the target is not as responsive as the others.

willie0912 : dit you change the capacitor with success ? Is there a capacitor on all your brief switches ?

I have found that you don't use a really cheap basic small ceramic disc for this application. You know the 50v small sized ones you can buy common values of 1000pcs for $3 from China which i can't remember the cap class code, but the most basic of cheap type. I had a mata hari game where those cheap caps lasted about a week or less before they started machine gunning. Once used a different type of capacitor with better specs it was fine.

The strobe/return pulses are not the same every game, every column/row. I think this stresses some pf caps more than others, but I am not for sure on that.

Thanks for your answer barakandl. I think there are different brands of capacitors on my Black Pyramid, even if they are all new. But it could explain my problem on the P target. I never thought that it could be a problem. I'll check this.

Very likely the diode on your "P" target is the problem. The "R" target on mine was doing the same thing, non stop scoring, wouldn't register as being hit. I replaced the capacitor first, no change. Replaced the diode and bingo, problem solved.