My T2 is having this exact same problem. I will be playing along and it will start adding a 2nd, 3rd and 4th player to my game as well as other issues. Looking at the switch matrix the start button and flipper button were in the same column (I think, or it could be same row). Upon further inspection I found that I was missing the capacitor on my left flipper switch. I am putting a new one on tomorrow and it should solve the problem. I believe if a regular switch is missing a diode it can cause the same behavior. Check all of the switches in the same row and column as your slam tilt and one of them may be missing a diode or capacitor depending on the type of switch.