Buying new boards is throwing away money. Which boards are you going to start with, and why?

As others have said above...Pam (who knows her shit) and Clay, (who knows his shit), this is in all probability a switch matrix issue. All the new boards in the world won't fix that. A slam tilt is (to me, and I've been at this a long time) one of the most difficult issues to track down. Once you track it down, it's a simple fix, but it's the getting there that's the problem. You have to take a systematical approach, and Clay outlined it perfectly above. Put the game into switch test, grab your manual, and close every switch on the playfield. Compare the switch number on the DMD to the switch number in the manual for the switch you are manually closing. If you find one that doesn't match up, your issue is probably on that switch, or three others....if the switch that doesn't match is 43, look at 43, 34, 33, and 44.

Please note that a "switch" error, as you described above, does NOT point to the culprit of a slam tilt problem. Switch errors mean two things: the switch has remained closed when it should have opened at some point, or that the game has not seen a given switch close in X number of games. That could mean you are a shitty player, or it could mean the switch is just shot, or the actuator isn't actuating, or a wire has come loose from the switch. You've cleared the switch errors (haven't you?), but you haven't touched the slam tilt issue.

One clue you found is that it happened when the ball in Sideshow. AFAIK, the magnet is activated by the left loop enter switch. The magnet will grab the ball, hold it, and drop it into the left (?) saucer. If the slam tilt occured when the ball hit the saucer, that is an excellent place to start. So start your switch test there. Enter switch edge test, close the switch, and see if the DMD reports the same number as what your manual calls for that switch.

Good luck.

forgot to say that is the switch test comes up ok, then look at every switch for a loose or miswired diode...where the silver band is not oriented as the others...the band will always go to a lug with no wires on it. Also, look at each switch, especially exposed ones on ramps, for any of the lugs touching other lugs or a lug touching a diode wire anywhere but where it is attached to a lug, little stray strands of wire that touch another lug or the diode wire where the strands didn't all get soldered to a lug, stuff like that.