Ok I can understand only the new heatsink part. Very possibly you had a bad (or wrong) heatsink that was not properly seating on the cpu core. Downgrading to pc-100 does not make any sense at all. You are losing considerable performance in terms of your memory bandwith by doing this and I would advise you know that you have the system up and running to go back to pc-133.
A little bit of knowledge is a dangerous thing!
A little bit of knowledge is a dangerous thing!