The problematic one I have is so unpredictable. Sometimes it will boot first time without issue, sometimes it will crash before it even gets off the black screen and sometimes it will randomly crash duri g the boot sequence. A handful of resets seems to be all it needs to get any game up and running, and once it's going it never seems to falter. I tired adding another 220uF cap to the rail near the pll but that didn't make any difference, I'll keep testing with various cap values when I get chance, but it's definitely a tricky problem.
Small update on this issue from me. I was mooching through the service manual and noticed VDP2 is responsible for the H-sync so I decided to replace the ceramic bypass caps for VDP2, I think it was C14, C76 and C77. It definitely hasn't fixed the crashing issues, but the fluctuations are almost completely resolved. There is still a very minor wobble when reading the CD but it's 95% better just off those 3 caps. I think I will do the bypass caps for the PLL to see if that helps with the crashing, and maybe just do a few other chips bypass caps with the same capacitance.
If anyone is having a wobble issue I would definitely advise giving the VDP2 bypass caps a go to see if that helps.
Returning to this topic after a brief hiatus, I had the VA9 open to revert the region and video output to PAL when I noticed something on the power connector going to the optical drive.
Removing this didn't fix everything as it still freezes with a disc in the drive, however it did allow me to boot the console with the lid closed which it couldn't do before. I have a hunch that replacing the optical drive altogether with an ODE could be a solution to this but that's a lot of money for me to put down when that's all I have to go on.