Welcome, Newcomer. Are you using SATA or IDE? If you're using a SATA, try the following:
1. Change the SATA data cable
2. Change the SATA power cable
3. Change the SATA port
In an effort to ensure your RAM is good, I would download MemTest86+
and burn the .iso file to cd. You must make a bootable cd. After burning, pop the disc into the problematic computer. For best results, test only one stick at a time.