Having one bad RAM slot in a PowerBook Aluminum is a common problem. Unfortunately, the only way to fix the problem is to replace the logic board. You can also try testing the RAM chips in the working slot to rule out a bad RAM chip.
Does anyone know why that other slot fails? I have the same problem and it seems like the other slot is getting ready to quit. It sometime don't boot up and I have to wiggle the other mem to get it to start.