Yaboot acting up

I'm noticing some strange behaviour in yaboot lately: when I switch on my laptop, there are times when it simply doesn't boot; instead, it gives me an error message about memory and something being too small (I'll write down the exact message next time if someone needs it to debug) and then drops me into OpenFirmware.

I can switch it off and on again at that point as much as I want to, but it will continue to fail. At one point it didn't even drop me into OpenFirmware anymore; instead, it looped over the first-stage booter all the time.

The only way I have found to make it boot again is to put in a bootable CD-ROM that uses yaboot to start, such as a Ubuntu live-CD. It doesn't even have to actually boot; just making sure I start yaboot from that CD once is enough.

I find this very strange, especially as the second time it happened, I didn't change anything that could remotely be related (the first time I did remove a kernel image). So before I start debugging my hardware: anyone else seeing this?