I set my mother up with a brand new Lenovo w510 with i5 and 4GB of ram running windows 7.
So there should be no problems running firefox or anything simple like that. What happens is that while she is using firefox, specifically while typing in text fields, she will type and the computer hangs for a second and then displays the typed characters.
I have personally seen it in firefox, but supposedly this also happens in chrome. Since she doesnt use much besides the internet, she couldnt tell me whether other applications yield the same problem.
Another thing to note is that this doesnt always happen, usually after the machine has been on for a few hours.