You're only setting isSprinting to false when you release the shift key, so, while you have it down isSprinting will be true, no matter how much stamina you have.
The code should be something like:
Keyboard | Shift is down | set isSprinting to True
Player | 8Direction is moving |
System | PlayerStamina > SprintVal |
System | Else | set isSprinting to False
Player | isSprinting | Set animation speed to 7.5
| Set 8Direction maximum speed to 375
System | Else | Set animation speed to [whatever the nromal speed is]
| Set 8Direction maximum speed to 250
System | Every 0.1 seconds | Subtract SpritVal from PlayerStamina
Player | isSprinting |
Hope this helps. Cheers!