Quoted from LTG:
Go over under playfield and backbox, check for anything loose. Parts, connectors etc.
This is very important. Stuff gets loose during shipping, may be missed during assembly, etc. Also check for any wiring that may get bound or pulled funny under the playfield.
When I get a new game (or new to me), I always replace the pinballs with brand new ones. This is almost a requirement if you buy a new Stern game.