What voltage does the CPU running at? Most DX2 parts were 5V ones but if it really runs at 100MHz it means that it has the 3x multiplier and is a DX4 indeed (it has 2x multiplier too - usually jumper selectable - so it can work as a DX2 too). What do its markings say?
There were 3V DX2s and I can easily imagine that these were actual DX4 parts 'binned' to DX2s.
If it's the case you should run the CPU at 3.3V instead of 5V but it must be printed on the CPU itself too. It should have an "&E3V1X" marking or something similar if it's an Intel part, simply "3 VOLT" if it's AMD, or something like "3.3 V" in case of a Cyrix.
Oh, and if it runs stable and at 3.3-3.45V it's perfectly safe to use at 100MHz (internal). At 5V it'll run much hotter than intended but may survive many years with sufficient cooling. I wouldn't bet my life on it though. Better safe than sorry.
There must be a FET close to the CPU that generates 3.3V from the 5V rail. Google showed that not every version of the board has it. And you must set it via the proper jumper too.
Shame on us, doomed from the start
May God have mercy on our dirty little hearts