A number of recent posts have queried the use of particular FireWire audio interfaces with SCS. Some are experiencing clicks and dropouts. However it seems that SCS may be blamed unfairly. It may be a mismatch between the interface and the particular chip or card in the computer.
The latest (December 2006) issue of Sound On Sound magazine has an article by Martin Walker in the PC Notes series which describes the problems can arise with particular chipsets and interfaces. In one case even a laptop video chip can cause a problem. Martin lists the websites that can provide information. If you are encountering problems and the article and/or the manufacturer of your audio interface indicates an incompatibility with your FireWire chip/card, you will need to replace either interface or card (if you are using a desktop). With a laptop the FireWire chip may be integral on the system board, but you could instal a PCMCIA-to-FireWire adaptor as a substitute. Martin also discusses Windows XP SP1 and SP2 issues and a program that can identify what chips you have installed.
For those who cannot obtain a copy of the magazine, an online subscription is available here. Individual articles may be released for purchased download in a few weeks when the next issue is out. It's a really helpful article.
Malcolm