XPS 400 SATA drives not always recognized

Dell XPS 400 P4 HT 3GBR 1TB Seagate Baracuda (primary drive - recent upgrade) 80 GB Maxtor (original drive) both HD run on SATA.

I've never had any trouble out of my computer until recently. The computer itself is almost 4 years old - not sure if that has anything to do with the prob. For Christmas I got a 1TB Seagate Baracuda HD to replace my aging 80 GB Maxtor (factory HD). I installed windows on the Seagate and things were going great. Last month, the computer shut down unexpectedly and when I tried to restart I got a message that no disks were found. I went into my BIOS and turned on all four SATA channels just to make sure the drive was turned on. That didn't work so I unplugged the SATA cables from the drives, switched them and it started right up. It's happened several times since then and now it's happening every time I restart the machine. It will fail to recognize the HDs - both in the BIOS and in the startup. I'll get to the DELL splash screen, and the hard drive lights will light up - 1,2,3, and just stay on. After a long time, it will go to a black screen that says something like, "SATA 00 disk not found, press F1 to continue F2 to enter startup utility. Usually when i unplug them and switch them it will fix itself again. USUALLY. Sometimes that doesn't even work. It's very strange - there doesn't seem to be a pattern to it. I just have to keep trying and restarting until it works.

ANY IDEAS?? I'm stumped...

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