I've solved my system wide stuttering with Mountain Lion by now (first sound only, mostly, then aggravated system wide).
Here's why: My MacBook Pro became unbootable from stock HDD.
I had a bootcamp partition, no luck when booting into Windows too, got stuck when booting, jusk like when booting into OS X (got stuck at apple sign with spinning wheel).
I've got a Ubuntu DVD and started a live session, SMART disk diagnostics: 258 bad blocks, imminent disk failure.
I half apologize Apple in this point, for blaming solely the OS for being just weird. The stuttering origins, in my case, wasn't solely about the OS.
I should have take notice since when first upgraded to Mountain Lion. With my stock HDD, everytime I tried to install OS X, I needed to reboot a few times and got negative time counter (WEIRD) for the remaining installation time.
So what confused me so much?
- I booted into Windows bootcamp: no stuttering, booted into Mountain Lion: stuttering plus very slow OS.
- OS X Disk Utility never helped or warned about the amount of bad blocks (doing PRAM/SMC resets, filesystem and permission repair never truly helped).
- When using Lion and Snow Leopard, there was no such a slowness/stuttering as there was in Mountain Lion. Before ML, I never experienced "sound stuttering", only when upgrading to ML this started to happen. So I suspect ML may be more sensible to disk with bad blocks.
I think I had few bad blocks initially and it became aggravated over time, I don't know whether ML usage helped in the aggravation process. Truth is, it was like I'd forced the disk to became unbootable when I last used it with Mountain Lion. It was slow as aways but still usable, I'd got a miniDisplayPort/HDMI cable and thought it was time to try it. The machine became very sloooow when I tried to play some video while connected to my TV, after disconnecting, it just hang, so I forced shutdown and it never booted again.
I've just replaced the original Toshiba MK5065GSX 500GB 5400RPM (what a crap and I think it was failing when under warranty) with a new Seagate Momentus XT 750GB (SSD hybrid drive, 7200RPM).
MAN, NOW IT'S BLAZING FAST! (also now I see the benefit of my early OWC 16GB RAM upgrade)
Anyway, I bought a MacBook Pro in early 2011 and here's what I've learned about MacBooks:
First problem:
MacBook Pro refuse to charge without any apparent reason, I thought it was some smart emergency battery calibration routine, no it was not. I've dumbly let the warranty end and didn't replaced this failing and VERY expensive MagSafe, needed to buy a new one. At some point the stock MagSafe stopped charging and didn't return to charge after some hours, as it was aways doing.
Second problem:
Slowness in Lion (I bought Lion)
Got RAM upgrade, no help (I bought 16GB RAM)
Got ML upgrade, NO HELP, got stuttering (I bought ML)
Died HDD (started dying around 1 year of usage)
Got new better HDD (bought)
Third problem:
Bad management of headphone jack makes sound speakers muted.
Blame OS X, any other OS: sound speakers working when expected to work.
Google for: Headphone jack red macbook
I think I can't solve this
Fourth problem:
I've spent a lot of more money
Given my experience, I have some concerns whether ML helps HDDs die.
Now it's blazing fast, no stuttering and I can't put a headphone. If a put one, sound speakers only return to be recognized at random afterwards.
I WILL NEVER BUY APPLE STUFF AGAIN
Maybe, only used and cheaper apple stuff for upgrades with my own choice of good hardware.
TIP: Aways check your disk healthy with serious tools
Best Answer
It is best to upgrade and then migrate. Usually problems occur while upgrading, and it is best to upgrade or install from scratch, as opposed to upgrade on a system which might have something incompatible already installed.