A sudden jump like that can only mean a few things:
The CPU heatsink dropped off?
The CPU cooling fan stopped turning
or somehow the heatsink has become unseated and not transfering heat properly to the heatsink via the (raised CPU core to heatsink base)
Has the case recieved a jolt? or has one of the retaining tabs on the CPU socket snapped off releasing the heatsink/cooler?
Just to put you in the picture 85C is absolute max temp for your XP2.4+ if your processor has genuinely reached 123C it would normally indicate thermal death for the processor.
Hope this isn't the case, good luck.
Martin
Replying helps further our knowledge, without comment leaves us wondering.