When i'm underground and mining blocks of any kind the blocks will re-appear unbroken multiple times, take 30 seconds to disappear and sometimes will not drop the resources (ie: cobble stone, coal, iron, ect) or experience points. sometimes the resources will appear a few inches from where i'm mining, then disappear and appear in the correct spot.
i've tried closing out and re-launching the game, as well as rebooting and launching the game; it continues to do it. this only recently started happening with the 1.6.2 update. here is a screen shot of the specs on my game http://i83.photobucket.com/albums/j320/Red2111/Untitled_zps42e178fe.jpg
I've also noticed that when this happens, Creepers and other monsters will move glitchy on screen