I haven't really experienced that before, really, and I've been through several of the G3 models. However, I did have two that did something similar to that.
First was a 700MHz G3 model. It worked, but would emit a loud whistling sound from underneath the trackpad. I'm pretty sure it was either a bad capacitor or maybe a bad inductor. I dunno, I used that one for parts.
The other was an 800MHz G3 model. It would turn on when it wanted, which wasn't very often. It was undoubtedly a power issue, but instead of playing find-the-bad-microscopic-SMT-device, I replaced the board. This was after I hunted for cold solder joints and outright bad components. Finding none, I decided it wasn't worth it to continue searching for something with so many possible culprits.
Both of those boards lacked the GPU problem, so it was unfortunate that they were bad. It wasn't all lost, though, as they both had the major ICs removed via heat gun; they're all now nice decorations.
First was a 700MHz G3 model. It worked, but would emit a loud whistling sound from underneath the trackpad. I'm pretty sure it was either a bad capacitor or maybe a bad inductor. I dunno, I used that one for parts.
The other was an 800MHz G3 model. It would turn on when it wanted, which wasn't very often. It was undoubtedly a power issue, but instead of playing find-the-bad-microscopic-SMT-device, I replaced the board. This was after I hunted for cold solder joints and outright bad components. Finding none, I decided it wasn't worth it to continue searching for something with so many possible culprits.
Both of those boards lacked the GPU problem, so it was unfortunate that they were bad. It wasn't all lost, though, as they both had the major ICs removed via heat gun; they're all now nice decorations.