SATA HDD intermittently loses connection

I am having a problem where an internal SATA HDD was consistently (every time) not being recognized by the BIOS of a Windows PC (I can't recall if it's XP or 7). I attempted to reseat the power and SATA cables and change the SATA port, with no improvement.

I placed the drive in a powered external enclosure and connected it via USB to another (XP) machine. The drive is only intermittently detected in either Windows Explorer or Disk Management, and it take several minutes on average to be recognized. I have only managed to successfully navigate to the Users directory on the drive one time before it lost its connection.

I would like to pull the data off of the drive, but don't know how to proceed. Has the hard drive already gone bad, or is there something that could be done to make the connection persist longer?

I assume by the actions above, the SATA cable, power connection, and motherboard can be ruled out as causes and the issue must lie with the hard drive itself.