Borderlands – My character has trouble running, he can only walk

For some reason every time I try to run by using the Shift key (and W) for example, my character just stops moving. I can't get him to run while walking by pressing the Shift key after the W key is pressed. If I press the Shift key first and then the W key, the character just jerks as if attempting to start the run, but then comes to a full stop. Repeating this just results in sort of jerks forward.

This seems to be some sort of bug. I can get around this for now, by jumping forward first, then mid-jump pressing the Shift key so that when he lands I already have Shift+W pressed. Then he starts running after completing the jump.

I am wondering if some config video quality setting could be causing this problem. Does anyone know of a solution to this to allow my character to run normally as he should.