First of all, I would like to clarify that by saying External keyboard, it means:
1. Bluetooth (not USB) keyboard on PC
2. USB and/or Bluetooth keyboard on laptop

I have tried searching the internet but haven't see similar situation, even though I have this issue on 2 different machines, Win 8 & Win 8.1.

I first notice the stutter when I play Sonic Generations & Dynasty Warriors 8 on my brother's Win 8.1 PC.

When I use PS/2 or USB keyboard and move the character by holding any of the W-A-S-D keys, the movements are smooth. Same goes for the other buttons.

Then he bought a Bluetooth keyboard, and the games are practically unplayable. Holding W-A-S-D would still move the character, but it stutters. As if I repeatedly hitting the key. Sometimes the character just stop moving even though I still holding down the W key on the keyboard.

The same goes for my Win 8 laptop. If I use the attached keyboard, I can play Counter Strike smoothly. But if I plug a USB keyboard and start the game, pressing W key would make my character stutter when walking forward, and even stop moving at all.

Would be very grateful if someone could help me.