I have a DeLock-branded, Si3132-based eSATA ExpressCard in my MacBook Pro, with the com.SiliconImage.driver.Si3132r5 driver 1.5.16 on 10.5.7.
Every now and then, it apparently decides to go nuts. All I/O to the external drive will be stalled for a few seconds, then continues, then stalls again, and so on. This has appeared a while ago, possibly with the 10.5.5 update. It appears to correspond to a system log message:
4/25/09 7:09:16 PM kernel SCSIPressurePathManager: new active path available, checking, loops = 0
And sure enough, 1) I know the driver works through the SCSI system (i.e., drives are faked as external SCSI ones, not SATA ones), and 2) the most useful Google result I could find has to do with ExpressCards. Other results suggest different drivers, but so far I've only found rebadged versions of SI's 1.1.9 driver (by Sonnet and by Rosewill), which exhibits the same problem. I assume this is some sort of caching bug.
One thing I have yet to try is to install 10.5.4's SCSI-related kernel extensions.
The stalling makes the card virtually useless, as performance via USB runs circles around this.
Update: as of 10.6.1 (10B504) / 1.1.11, this is still occurring.
Those links may help you :
http://discussions.apple.com/thread.jspa?messageID=8628956
http://www.snsforums.com/index.php?showtopic=250
http://forums.applenova.com/showthread.php?s=3bbe55948d64dca96ea3ced8d1905626&t=32027
But after many search, there still no clear solution to the problem up to right now...
Hope it may help you but it's seems a very common problem with no known solution...
If upgrading your driver isn't working, you could try downgrading it instead. The vendors website has older versions available:
http://www.siliconimage.com/support/searchresults.aspx?pid=32&cat=3