From your description, all your trouble is with 8G Kingston ram. LIon and M. Lion are more demanding on ram quality. Buy Crucial, or the same as the original brand. Pay close attention to the speed, it has to be the same as original. I have the same lappy as you and first I had it with 7200rpm, than SSD. Boy that puppy is fast and stable with all first SL, than Lion and now ML. Avoid Kingston memory at all cost. Also flash from them is a real ****.
The VEREDICT: I HATE ML!!!!
First of all, my machine specs:
Macbook Pro Mid 2010
i5 2.4ghz
500gb Black Scorpio
8gb Kingston
CLEAN version of ML........... TWICE. (Did it twice to check if the installation was the problem)
Q: What do I mostly do with my mac?
A: Word, Excel, PDFs, Firefox, Mail.
Q: Why do I HATE ML?
A: For the same reason I hated Lion: Firefox crashes every single day (I tried Safari and Chrome... same ***** too), computer got a little bit slower than SL, takes more time to open apps.
The worst part is when ML freezes the browser (even SAFARI!!!): NOTHING else works the same. Its like running a 486 PC with Mountain Lion! Really slow. So I have to reebot it. Pretty good, uh?
Funny thing is that it happened with Lion... the same way!
So, Apple, since the same happened with Lion, the solution would be:
A. Buy a brand new Macbook Pro or;
B. Back to a 2009 operational system (Snow Leopard).
Right?
IMPORTANT: IF YOU ARE A FANBOY, PLEASE, no whining here.
I will be back to SL tonight. At least its FAST and doesn't crash. Simple.