Stuttering problems are usually always a case of something interfering with the games access to the HDD, or not having a multicore system.
The game streams in its content using a seperate thread, and if something is delaying/occassionaly blocking the streaming (or there is no such thread available to the system, or something else is using the thread), then you will have stuttering as necessary data is loaded in too late. The game then starts resolving to all kinds of emergency systems, leading to degraded graphics quality and performance.
In our office, streaming problems are usually related to the virus scanners or other "resident" applications, and when we switch them off, the streaming problems disappear. Fragmented HDD's are another cause as is running some other application in the background that takes up performance.
