Does this "bad" file actually play ok? (i.e. before trying to change the tempo).
Select it, in the Project window, then, Audio menu>Bounce Selection. This will create a new file, hopefully, stripped of whatever it was that was making it behave strangely.
The bad file DID actually play ok. But I am unable to try your "Bounce" technique on it because now when I try to open it, it says "Cubase Has Stopped Working" then crashes. Luckily I was able to open it previously and renamed it. So everything is working on the new file.
One last question... I see you guys have "been thanked" meters next to your names. How do you "thank" someone??