Is the shield present over the transformers? Apparently, a wave in Tapper cocktails is not uncommon. I guess that it draws a fair amount of current, causing a more powerful magnetic field from the transformers than other games. I have seen one in person that this as well. Didn't matter if the shield was there or not, although it was much worse with the shield off. My Super Pac cocktail has some wave in it, even though the shield is present.
Another way you can check is to power up the cabinet with the top open. I believe it opens the same way as the others. the monitor should be attached to the top. If you fire it up with it open, the magnetic field should not be able to cause a wave. Might get some gauss effect with it open but if the wave is caused by the transformers it should stop.