Drives are disappearing.

I have an XPS9100. My configuration is Win7 Ultimate (64-bit), 9GB Ram, Core i7-930

WD 1TB hard drive on C (SATA Port 2): Contains OS and a few main programs (MS Office, etc.)

Seagate 1TB hard drive on X (SATA Port 3 as of today): Contains my data (My Docs, Outlook pst, etc.) and secondary programs (games)

Seagate 500GB hard drive on Y (SATE Port 4 as of today): Contains storage (secondary backup of photos, videos)

TSST DVD on D (SATA Port 0):

Media drive (no drive letter shown until something is plugged in.)

Today I started losing access to my X-drive. It just wasn't there in Windows. Since I first got the computer, over a year ago, whenever I ran the Dell Support Center diagnostic, the X-drive would disappear. A reboot would generally bring it back and I didn't address it when I had tech support because I thought, "Eh, I'll just never run that diagnostic." Shoulda, woulda, coulda.

Today, a reboot did not bring it back the first 2 times (I checked the BIOS setup, and the drive appeared there, but it was not there after Windows startup, and did not show in Device Manager), then on the third reboot (panic in full force, and I ran the Dell Support center hardware diagnostic with no errors other then the X-drive not being there) it did appear normal after completely booting up, only to disappear after 10 minutes of inactivity (computer did not sleep, but maybe there was some background stuff going on??). I cracked the case, and swapped the SATA cables connecting drives X and Y, so that they were now on different ports (as seen above) and the cables were now different. Problem solved ...ish.

My computer was SLOWWWW. I had all sorts of issues trying to get any right-click functions to work. Clicking on Windows Explorer in the taskbar brought up the window almost immediately, but hitting Windows+E had a long (3 minutes+) delay. I ran Norton AV scanner. No issues found. Malwarebytes. No issues found. Ran Windows System File Checker  (SFC /verifyonly). No issues found.

Pretty much as soon as Norton was done, a warning pops up from the taskbar notification area from the Intel Rapid Storage Technology app (which came installed on the system), telling me that Port 0 disconnected. Sure enough, no DVD drive in Windows Explorer. I opened Device Manager, scanned for new hardware, and the drive reappeared for about a minute before disappearing again. I tried scan for new hardware. No joy.

So, same day, two different SATA drives disappear. What is going on here? Hardware conflict? They don't use IRQs anymore, do they? Failing controller on the MOBO? Failing power supply?

BTW, the incredible slowdown of my computer (responses to right-clicks and Start Menu selections) seems to have abated. Maybe the system was stuck doing a lot in background to figure out what how to handle the switching of the ports or something. I'm just grasping at straws here.