it's a fun game. but if too much is going on on the screen, it bugs out. nothing more frustrating than having enough money to buy stuff or upgrade...and the game controls freeze and you have to watch while the zeds eat you alive.
until level 38 it's ok. at that point it gets worse and worse
level 44: unplayable
ok. got it. you have to hit "P" (pause) for the game to get a grip again.
hit "P" again, then you can execute some stuff. like three things.
reached level 97 with this technique.
annoying, but it works.
would give 5 out of 5 stars if this problem was solved.