[Faster]

Sounds like you need at least one new coupling half, possibly a new shaft (axle isn't the right term here!)

 

[Faster]

Thanks Faster. Your right, axle is in bad form. (I knew something looked off when I wrote it and didn't really proof)
So the coupling is causing the key to throw? Hows that?

Hard to say without pictures, but if the keypiece is 'coming out' then either it was the wrong size, or the coupling-half and/or the shaft are worn from working against each other when improperly tightened.. either way one or both may be damaged to the point that you can't properly reassemble it all.

Join Photobucket, upload your pics there and paste the IMG CODE link into your post so we can see what the issues might be.