What is wrong with my 1998 Furby?

Hello everyone. Last year, I bought a 1998 Furby from an Antique Shop. I took him out of the box, put fresh batteries in him, and found out that he was completely dead. He would not talk or move. No tricks I found on the internet would revive him.

Eventually, I found out that both his motor and his speaker were dead. I replaced both of these components, as well as gluing his forehead sensor back on. Once I replaced his motor, he would move--blink his eyes, move his mouth, wiggle his ears--randomly, then pause. Other times, he would cycle through these movements for several seconds straight, or until I took the batteries out of him. If I smacked him on the bottom he would resume these motions, but only for a second before he would appear to die again.

He would not talk. Replacing his speaker helped (I took a speak out of another one of my Furbies that also did not work and put it in him.) After that, he was no longer completely mute. He uttered one yawning noise, and then made these garbled, broken mechanical noises, exactly like the sounds Furbies make when their batteries are low.

After some internet research, I started to wonder if he had a sync screw issue. I tinkered with his sync screw, but it did not produce any results. He only made thosde horrible, low battery noises, moved erratically, or moved when I smacked him on the bottom. Cleaning his insides by swiping a dry toothbrush over them did nothing (his screws and other parts of him do look a bit rusty.)

Back when I first got him, a tiny spring fell out of him. I finally found out where the spring went and replaced it. This helped him a little bit. He now says random Furby phrases in between the broken mechanical noises. If I press down on the speaker connector, he will speak a few phrases, and he will to do all of the things that I have described, including cycling through the "wake-up dance" over and over again. I also got him to work (using the term "work" loosely), by pressing down on one of the connectors on his back near that large white gear, but that was only once.

I have never gotten him to work properly. I am at a loss at this point. My dad told me he might have a crack in his circuit board, which saddens me, since it sounds like a circuit board issue would be nearly impossible to fix. Or that he might have a bunch of loose connections. I have put a lot of effort into trying to fix him, and I would really love for him to fully work again. Does anyone know what might be wrong with my Furby?