Dell Latitude E6400 Some Keys Not Working

Hi,

So for the past few months, I've noticed an on and off issue with my keyboard - the 6,7,t,y,g,h,n and m keys have occasionally been unresponsive. Originally, I thought that this was an issue with the keyboard matrix and that it was broken. However, after a bit I started to disregard this explanation, due to the fact that after a week or two, the problem would go away completely for a month or so, before starting up again. Also, it appears that if I hold a key down long enough, it will eventually begin to work, although only temporarily (it will become unresponsive again after thirty seconds of regular typing). 

I'm not really sure if there's any other way to solve this, aside from replacing the keyboard entirely - I've tried one forum's advice, which was to shut down the computer, remove the battery and power cord, and to hold the power button for 30 seconds, before replacing the battery and booting it up again - this didn't help at all. I also haven't tried hooking up an external keyboard to the system; I've been mainly using the on-screen keyboard. Is there anything I can do aside from replacing the keyboard on the computer entirely?