Check your time compression settings in MyDocuments ...\SH3\data\cfg\main.cfg.
Open with notepad and ensure RealTime=1
Do you use the SH3 Commander?
8Gb RAM? So I assume you have Win8 64bit?
Do you have a 64 bit driver for your graphics card?
http://www.nvidia.de/Download/index.aspx?lang=en-us