I finally have the sling shot working 100% of the time. Later i will post the capx file.
To me it seems that the physics impulse is not being executed (meaning i'm not waiting enough ticks for it to execute.) So all i did was make the impulse condition true until the blue box is overlapping the brown cirle. Once it is overlapping make the impulse condition false.
What i don't know is how many impulses are being executed.