Problem with using Lshift to run in MHFU

[XionZion]

So I recently mapped the keyboard/mouse controls and was pretty happy and comfortable with the result. I mapped the R key to Left shift so I could run "classic" style, but if I hold down the key, it only makes the character sprint for a little while, and then stops. Meaning I can only sprint for about half a second, which is kind of annoying. Anyone know a fix? Should I map the keys differently?