You are a little confused. PC2100 RAM is PC2100 RAM - it doesn't come in different speeds - so different speeds cannot be the problem.
Most likely you have done something when you made the change.
Did you unplug the system from the mains, or switch off at the mains, before changing the RAM. If you did not, then you may have permanently damaged all of your RAM.
Wothout you remove mains from the system, there is still power on the RAM sockets - switching off at the frone is not sufficient.