Dell Inspiron 1501 keyboard problem

Hi, I've had my dell inspiron 1501 for over a year now, it's been well used in that time. I now have a keyboard problem with the laptop. When i'm typing (in any program, may it be a text editor or web browser), I can type a few letters (I type pretty quick, as fast as anyone that uses a computer most days) then as I'm typing the letters start coming up very slowly, and the curser moves along ALOT slower than I'm typing. It appears to be lagging or something. Sometimes the curser jumps and a load of letters appear at the same time.

I've tried using an external USB keyboard and the same problem persists. I've even tried wiping the laptop. Occasionaly, I boot up the laptop and the problem doesn't occur until i reboot again. I've just wiped it using dell system restore, and the problem didn't occur so I thought great! It's gone. Then I rebooted and it reoccured... Thanks in advance for any replies.