Serious boot issues with Powerbook G3 (Wallstreet II, OS 9.2.2)

Hello,

I have major issues with a Powerbook Wallstreet II I got recently. When it's finally running, it works just as well as Wallstreet Powerbooks usually do, no errors, no crashing, smooth performance, everything fine.

But booting it up is, to put it mildy, a bloody pain. First off, it does not power up, when it's only connected to batteries. I have two of them, both working fine for several hours when put in after bootup, but no matter if I place one or both of them in any bays, nothing happens when pressing or holding the power button (apart from screen flickering and speaker crackling). I'm always forced to connect the power adapter, which immediatly (without pressing the power button) causes the power LED to light and the cooling fan insanely spins up for a mere couple of split seconds until everything wents silent again.
Having reached this point, I can disconnect the power supply (if at least one battery is in) and usually hear a nice "BONG" when pressing the power button once again. Thereafter, nothing happens for a oppressive 30 seconds, then, the screen finally lits and shows the Mac OS 9.2.x splash screen on monochrome background. Sadly, this is already the end of most of my first boot up tries, because at this point, I usually get told by a dialogbox that a bus error has just happened. The box also has a nice "Reboot" button, which I cannot click as the trackpad hangs as well.
Pressing Shift Fn+CrtlPower works here in about one out of three time, so in most cases, I have to go through the same procedure. Pull out power sources, give a shot via PSU, have it iddling for 30 secs. In most cases, this second try lets me pass splash screen, loader, yes, it even lets me see the desktop, the taskbar, the HDD icon and... pouf. Success is so close but no, it has to suddenly drop into sleep mode and
a short press on power proves the apprehension true, that it won't be easy to wake it up again. Hrmph. But I don't give that easily. So pulling everthing out for the second time, connect PSU for the third time and look there, it fully boots.
Three tries to successfully boot is about average at first try series. Afterwards, it seems to be "warmed up" and things get easier. I made a log recently with 5 test series, made at very frequent intervals:

Series 1:
1. Bootup: Bus Failure (Hard reboot fails) (bays empty)
2. Bootup: Sleep Mode (bays empty)
3. Bootup: Sleep Mode (bays empty)
4. Bootup: Sleep Mode (both batteries)
5. Bootup: Bus Failure (Hard reboot successful)(both batteries)
6. Bootup: Success (left battery, right bay empty)

Series 2:
1. Bootup: Success(left battery, right bay empty)

Series 3:
1. Bootup: Bus Failure (!!!) (left battery, right bay empty)

2. Bootup: Success (bays empty)

Series 4:
1. Bootup Success (left bay empty, right battery)

Series 5:
1. Bootup Success (both batteries)

Thanks in advance for any helpful suggestions.