Shredso, if you are testing the pitch during a 1 player game...the machine will randomly choose the pitch to throw regardless which pitch button you select. That may be the reason you are witnessing such a random pitch...its part of the strategy of the game.

You could play a 2 player game, then each fastball should head straight across the plate.

When I picked up my Slugfest, the buttons were wired wrong causing curve balls instead of fast balls...I wouldn't rule out that possibility...or something similar.

Finally, you could always disconnect the magnet under the playfield and test the pitch, that would rule out everything except a playfield needing a level.