Performa 6116CD Startup Problems

Hi, I tried posting this earlier, however Apple’s discussion web site went down. I have a Performa 6116CD computer that we’ve owned for at least 13 years. It was our first computer. I am trying to get it started so I can use it again, however I am running into major problems. The computer has System 7.5.1 installed on the Hard Drive, has 40MB memory, CD drive, Floppy, and PowerPC 601 CPU. We stopped regularly using this computer in 2001. The last time I tried using this computer was in 2004, and at the time it refused to start up and only showed me a Flashing Question mark. It has been stored since then. I set it back up and tried starting it. I received the same Flashing Question Mark. I tried shutting it down and restarting it several times. Each time, all I would get is the flashing question mark or a smily face Mac logo and then it would freeze. I could hear the Hard Drive clicking and spinning like it wants to start up. Each time, it started clicking and then it seems as if something steps in and stops it from loading. It stops clicking and goes silent. I started it up using the Performa CD. It did not even recognize the Hard Drive. I suspected the PRAM Battery to be the culprit of these problems. It is the original battery and must be at least 13 years old. I replaced it and made sure the + side was in the correct position. I started the computer back up, and the first time or two it acted the same. However, on the third or fourth try it got past the smily face mac/question mark and actually started loading the OS. Almost every icon at the bottom of the screen loaded, and then the computer froze. I shut it off and restarted it to receive the same flashing question mark. I restarted it several times and even reset the PRAM. I pressed the reset button on the back, but that did not help. Finally, it starting loading the OS again. It completely loaded the OS, and I was actually able to use it. However, the system was slow and sluggish. When I tried to eject a CD, the computer froze, and I had to restart it. Once again, the dreaded question mark appeared again. I pressed the Interrupt button on the back, and I received a loud crashing noise through the speakers. I turned it off and loaded the Performa CD. It recognized the HD and asked if I wanted to initialize it. I said yes, and then it said the HD was unable to be formatted and was unusable. I tried Disk First Aid and HD SCSI Setup, and both said the hard drive was unusable. I shut the computer down and tried to isolate the problem. I unplugged the CD drive’s SCSI cable, power cable, and audio cable. I removed the added memory chips, except for the built in memory. I unplugged the printer, keyboard, and mouse. I even plugged the SCSI cable’s second connector to the hard drive instead of the connector at the end of the cable. I tried booting it up, and all I got was the same question mark. The last, final time I tried booting it, I didn’t even get a question mark, just a background image. At first, I thought the PRAM Battery was the problem, now I’m convinced that it’s either a Bad Hard Drive, Bad SCSI Controller, or Bad Logic Board. Also, this hard drive is not the original. The original hard drive went bad back in 1999. An authorized Apple repair shop replaced it with an identical Quantam 700MB SCSI hard drive. In 2001, we stopped using this computer on a regular basis. Since then, it’s mostly been stored. If the HD was bad, I would think it would not even make any noise, click over, or act like it wants to load the OS. I’m trying to avoid spending a lot of money on an old computer. What do you think could be the problem with this machine?