Movement input problems

[Drunk In Space]

I'm using keyboard/mouse to control things, and I seem to be having issues with WASD movement in the game. When moving in any direction from a stop position, the character instead does this kind of stuttery limping motion. I can fix this by pressing another movement key, but the problem reoccurs whenever I stop moving and start again.

This is annoying, but I can kind of deal with it when walking/running around. However, it's a much bigger problem when driving. When pressing the W key to move forward, nothing happens, so I have to press another movement to make the car go foward. Unfortunately, every time I take my foot off the gas, it resets and I lose input control, so the car effectively stalls. This also affects the A and D inputs, making it difficult to steer the car. Overall, driving is almost an impossible task because of these problems.

Anyone know what could be causing this? I don't have any other performance issues (such as the slow-motion bug some people have been talking about). I've updated my drivers, checked my game integrity, and made sure nothing's wrong with my keyboard (no other game has this issue). I've tried tinkering with various video settings, such as turning off vsync (which actually made the problem worse), adjusting resolution, going to windowed mode etc, but nothing seems to help.