Same issue
My girlfriend got a second hand iBook G4 that had been through a quite a bit before she got it.
A few months after she got it, the computer slowed down considerably. Granted it still had old files tucked away in the Library from the previous user and she was using up the computers 30 GB pretty fast but it was abnormally slow. I checked it out and saw that it was only recognizing the factory installed 512MB RAM chip. We discussed getting a 1 GB chip to max it out but a few weeks later it started running faster again after an update and a restart so we left it alone.
I decided to take a look at how much RAM it was running a bit later because I had forgotten the type of chip. When I checked in the "About This Mac" it said it was running on the full 1.5 GB. I was shocked and checked the RAM chip itself, taking all the precautions to avoid static, and there was already a 1 GB chip in there!
That was about 6 months ago and the computer is doing it again. It is reporting a failure in the Memory slot itself on the power on self test too. I checked the position of the chip to make sure it is secure but the computer doesn't seem to be recognizing the memory slot properly at all.
I am going to do Apple's Hardware Test and see if that helps!
