I hope someone finds a solution to this; it started happening on my Win7-64 machine tonight too. RAID1 1Tb. Exceedingly slow. PC is now pretty much unusable. HDD light on solid of course.
Had same problem, but is now fixed. Had Win 7x64 installed on SSD and small RAID 0partition and bulk RAID 1 partition on pair of 1.5 TB drives. Noticed that when drive was rebuilding and rebooted, the Intel RAID bios splash screen would say rebuilding. If I had rebuilt the mirror when I rebooted, it would say that all drives were fine. Whenever it would then boot into Windows, it would take an eternity, and when it loaded, the Intel software would say that the RAID 1 partition had degraded and the RAID 0 partition had failed. Thus, the cycle would start again. Even tried playing with disabling the HDD cache, but that didn't help.
After reading about installations with Win 7, I took a chance and reinstalled the OS. Before I did it, I downloaded the latest Intel Rapid Storage driver, and inside was a folder to make a driver disk (any USB flash drive will do). I did a clean reinstall (kept backup image in case this didn't fix anything), chose custom install, and loaded the driver. Install went off normally. Now, the drive never fails. Rebooted dozens of times with no problems. I'm confident the problem has been fixed.
Hope this works for you!