Hi guys,
I have been having this problem in the last couple of months, I have Ubuntu installed on my machine for 6 months now, and all of the sudden the following keys:
-Alt
-G
-H
-Arrow UP

They intermittently stop working without notice, I can log into Gnome and use it no problems but then after a minute or two..these keys do not respond, this computer has some sort of mouse in between the G & H keys, I thought it would have to do with some garbage, but is not, I can rebook the PC and the keys would not work...and some times the key start working again... I cant find anything as bizzarre on google, can you please let me know how to troubleshoot this issue.

THank you all in advance.!

First of all, change your password and root's to make sure you wom't need those keys to log in.
Examine the keyboard options in xorg.conf, and check numlock, right-windows-key or some other strange modifier is affecting things.

Electronics wise, keyboards have a matrix; they power, for example one horizontal line, and sample the verticals. All will show power except the keys pressed. They were half a row last tiome I looked at it, so your symptoms would discount a hardware problem. But there are all sorts of fancy key mappings in .Xmodmap files and elsewhere, and try getting .Xmodmap out, going to en_US locale, and see if that helps. Otherwise it could be dodgy or dry jointed key buttons. Best test for this is to press a key when they're not working, twist it while pressed and see if it comes alive.

Idd I'm thinking of a hardware problem too, the way you mention it, as an addition to business_kid's post, try connecting an extra keyboard. If the keyboard on your laptop stops working, but your external still works, this could be the issue.

There's an other thing with some laptop keyboards. Some have a sticky Fn key wich will disable some keys temporarily. If your Fn key is close to your shift key, try pressing it when the problem occurs. Maybe it will help.