Keyboard issue - key presses occasionally don't register

I have an Early 2011 MacBook Pro, 15" model. In the past few months, it has been developing a problem. I will be typing, and the letters appear instantaneously as usual (not delayed, as happens when the CPU is bogged down), but some of the letters get skipped.

"Hello thr, Tom, how are ou? Enjyng the sun tody?"

I have to delete, and then try again to type the letters. It skips totally random letters. While typing "Hello there", I might get "Hllo" then delete and on the second attempt get "Heo", then on the third attempt get "Hell there", then finally get it right.

It is annoying. Is this a symptom of the CPU being bogged down, or is this a keyboard problem? What do you think?