So, I finally decided that it was time to upgrade the software on SpiffMAME. I really hope some day I can get everything up and running with the latest versions at the same time (including newest AdvanceMAME, romset, an updated Gentoo, and a stable configuration). If that ever happens, I think I will "freeze" the config (perhaps make an image onto a different partiton, so I can revert if bad things happen). But so far I struggle to get different things updated when I have the time.
After taking the computer out of my cabinet (I have the mother board, harddisk and power supply mounted on a piece of particle board - no PC cabinet), and hooking it up to my monitor, I discovered why the cabinet had taken so long to boot the last couple of times I started it up. Aparently there was a problem with my attempt to make LILO switch between maintenance mode and normal play. This resulted in a 60 sec timeout before the system started booting. This has been reverted back to a 5 sec timeout, until I come up with a better solution (perhaps something involving my new homemade keyboard controller).
When the system finally started up, there were a lot of errors. It turned out that there were errors on the filesystem (crosslinks), so the system was having a hard time even starting up. I assume (and hope) the errors were related to the system not being cleanly shut down (it has crashed on me a couple of times, most likely due to bad AdvanceMAME configuration etc).
After fixing the filesystem and rebooting, only one error appeared, related to some ALSA-library, so I hope this will be fixed by recompiling the ALSA subsystem (after I upgrade the kernel).
Right now I'm waiting for the emerge sync to finish. After that I can upgrade packages.
Then I will upgrade the kernel. The newest kernels support rotating the console on the framebuffer, which I want to experiment a bit with. It would be extremely cool with a readable console on a vertical monitor (avoiding the neck-breaking practice of using it).