I'm thinking a power problem. Particularly if the hard drive shuts down when in use.
Try using a different Molex connector. Preferably one from a different "tree branch" from the power supply.
Also try unplugging any non-critical 12v devices on the motherboard and test.
If the power supply is as old as that IDE hard drive it could be in it's last days, and adding the SSD pushed it over the edge.