Why is my DC motor losing torque when over voltaged
In a nutshell for a PMDC motor:


voltage = speed 
current = torque


Note that for a given voltage there is a speed at which there is (almost) no current going through the rotor, because of back emf (the rotor is inside a magnetic field generating a counter-emf on the brush inputs).  To get more torque at that speed you have to increase the voltage (and basically overdrive the motor, leading to early failure).

There are a whole bunch of other factors that can come into play such as mechanical resonances at certain speeds, etc.

Regarding the startup problem you had/have- is it repeatable or did it just happen the first time?  It could have been a poor brush contact, etc.

Gory details:

http://lancet.mit.edu/motors/motors3.html