You can't just overclock to some given frequency and expect it to be stable. At a certain point-- usually not that far from stock-- you need to start manually adjusting voltages. A higher frequency requires higher voltages, and higher voltages increase heat. Your system probably isn't stable because you're not giving it enough voltage somewhere, and that's only showing up in applications like Motion that can really peg the CPU.
99.9% of the time if going from one OS to another breaks your overclock... (but the system will run fine at stock in both systems)-- that's a sign that your system was never really stable in the first place. It's just that the new OS/apps are pushing your system harder (maybe getting more out of them... a good thing!) than before, so that instability is finally showing up. That's why people who OC tend to test for stability by running apps (Intel Burn Test/Linpack/LinX, Prime95, Handbreak, FPS gaming) that are able to variously peg the CPU, peg the GPU, peg the RAM, and raise heat-- ideally to a degree beyond what any other application you're going to use will do. When it's stable at those things, then it's ready for day-to-day use.
Too fast with too little voltage = crashing.
Too fast with way too little voltage, or too fast no matter the voltage = won't boot.
Too much heat = automatic shutdown (to protect the system).
Too much voltage = damage.
TANSTAAFL!