More info about compatibility. my machine MacBookPro5,2
MacBook Pro (17-inch, Mid 2009) - Technical Specifications
1067 MHz ram.
8GB kit (4GBx2) DDR3 PC3 8500 1067MHz for your 2009 / 2010 Apple Macbook Pro & iMac
MacBook5,2. Those MacBooks, which still feature the legacy pre-unibody case design and were manufactured in early- to mid-2009, are supported in Mountain Lion because they feature an NVIDIA GeForce 9400M graphics card instead of the integrated Intel GPUs in their predecessors.
Mountain Lion will only run on the following Macs:
iMac (mid 2007 or later)
MacBook (13-inch Aluminum, 2008), (13-inch, Early 2009 or later)
MacBook Pro (Mid/late 2007 or later)
MacBook Air (Late 2008 or later)
Mac Mini (Early 2009 or later)
Mac Pro (Early 2008 or later)
Xserve (Early 2009)
Not be able to run Mountain Lion (model identifiers in parentheses):
Late 2006 iMacs (iMac5,1, iMac5,2, iMac6,1)
All plastic MacBooks that pre-date the aluminum unibody redesign (MacBook2,1, MacBook3,1, MacBook4,1)
MacBook Pros released prior to June 2007 (MacBookPro2,1, MacBookPro2,2)
The original MacBook Air (MacBookAir1,1)
The Mid-2007 Mac mini (Macmini2,1)
The original Mac Pro and its 8-core 2007 refresh (MacPro1,1, MacPro2,1)
Late 2006 and Early 2008 Xserves (Xserve1,1, Xserve2,1)
Update: It slipped my mind that there were two pre-unibody plastic MacBooks introduced between the aluminum MacBook and the plastic unibody redesign, both of them with the model identifier MacBook5,2.
Those MacBooks, which still feature the legacy pre-unibody case design and were manufactured in early- to mid-2009, are supported in Mountain Lion because they feature an NVIDIA GeForce 9400M graphics card instead of the integrated Intel GPUs in their predecessors.
Taken from here
The fact that even identical models work/don't is tripped by "hardware" and 3rd party influences, pretty sure of that.
There are cases of lag going back quite a while, not sure if its related to todays issues.
Lets hope the fix is quickly released. [solved] is good to see.