I would replace the cap and rotor.
My MSD setup has left me stranded on the side of the road twice in 2.5 years and both times I had to put a new cap and rotor in to get it going again (the first time obviously I put a new pickup in, replaced the coil, checked the box, rewired the ignition, all to find out the cap went bad.
Both times...just driving down the road the car simply shut off.
Now I have an extra cap and rotor in the glove box at all times...which obviously means it will never happen to me again.
And yes, if you short the control wire and it fires a spark the box is fine.