After playing about 10 minutes with no problems, the PET disappears so I can't interact and can't even exit properly. At that time, the rest of the graphics also start stalling. What might be causing this problem? Can I fix it?

What's your running system ? Do you have Windows 2000 installed? (rather bad)
Is you Direct X at the latest?

Have a look at the Tech.Support, perhaps something there would match your problem.

http://www.protext.pwp.blueyonder.co.uk/sst-forum/techsupport.htm

greetz Capt.Ed