Without more information about the specs for each system, it's difficult to say. Based on what you've written, the only technical statement is that above 512MB RAM, Win98SE becomes unstable. As a simple test, try taking out one memory module. Otherwise, we need more stats to help you.
Second, what version of the Performance Tool are you using? Do you have the latest update for 1.1?
Second, what version of the Performance Tool are you using? Do you have the latest update for 1.1?