Well, first off, this is most likely an issue with YOUR system, not the game. If it was the game, with hundreds of people playing it there would have been a lot more people posting about this issue.
A couple things to look into. Be sure your keyboard is plugged in well to the computer. They do loosen up if the box gets moved/knocked about.
You might want to try another keyboard if you have an old one lying around, yours may be dying.
With it being so intermittent it might be hard to narrow down unless you do a lot of typing and work with your number pad in say "word" or some other program.
Also, just as a note, AH2 and AH3 are completely different games and can only be compared with that both have planes in them. AH2 was very dependent on the CPU, were as AH3 puts to load on the graphics card instead.