See if that is the way windows is reading the input. In Win 7, go do your devices + printers in the start menu. Then find where the gamepad device shows up and right click and go to settings. click on your device and it will bring up a diagram of what you push. see if pushing left shows as a constant press or going on and off repeatedly.
If it shows a constant press as normal, I would switch to FCE Ultra as my NES emulator, see if that works without problems.