I have a non working left slingshot on my Bally Kiss.
I changed the Q10 transistor and the diode. Then I changed U3 chip.
I tried with a board from my Bally Star Trek and the left slingshot works perfect.
I changed back to the broken board and suddenly the left slingshot worked for about 5 minutes then dead again.
I grounded the Q10 and the coil fires. Seems the problem is on the board, a glitch?
What should I try next?

Yes I also think a bad connection. But I measured J5 pin 15, that goes to left sling coil and that measures fine but wont work when slingshot switches are closed. Which connection on solenoid board receives signal from mpu board? Could that be the connection/pin that is bad?
(I am a bit into replacing U2 and maybe U1 just to see if that is the problem, or is that overdoing it and unnessesary?)

Thanks guys for all the help. I now measured J4 and followed the traces and everything seems okay when the board is out of the game.
I put it back in the game and left sling works for about three credits. I take of the glass over playfield and backbox. When pressing sling switch nothing. I bend J4 some - nothing, I press U2 some, then I press U3 and the sling coil fires. So I have now found the area where the glitch/bad connection is. If I bend the board against me, the sling doesn’t fire. Tomorrow I will take it out of the game and check around U3.