IBM PC300GL rebooting problem…

We have two IBM PC 300GL Type: 6287, Model: 61U. Both of these machines within a period of 3-6 months after purchase, starting rebooting itself in Win98. You’ll be working on a Excel spreadsheet (or any program) and it just reboots by itself unexpectedly. I’ve replaced the CPU, HD, DIMMs, Network cards, reformated the hd and re-install Win98 from scratch, etc., etc… nothing works. I know it’s a motherboard problem. IBM will be sending me new motherboards replacements, but I just wanted to askyou folks if anyone of you have similar machines that does the same thing… or anything else to suggest?