simple answer: nope
long answer: overheating northbridge contains the mosfets, these play a significant part in keeping your system stable
if its overheating even though it's waterblocked then you've done something wrong
most of the time the northbridge can be sufficiently cooled by applying epoxy and gluing a passive copper block onto it, northbridge/southbridge blocks are pretty gimmicky...
if ur northbridge overheats then ur system crashes more often, some mobos have a downclocking option as a safeguard after crashing say three times in a row due to an unstable overclock, i believe the asus mobo's have this function
hence your thoughts about downclocking due to a mosfet problem
downclocking can also be intel speedstep doin its job, is ss enabled in ur system? ss just downclocks ur system to relieve heat made by ur cpu, nothing to worry about
hope this helps