afremont:
I assume that the overflow ISR is doing something with toggling pins based upon the comments.
Yep, you assumed correctly. The code is used to drive a stepper motor which needs an alternating HIGH/LOW sequence of pin actuation.
Yep, it was misnamed, must have happened while I was doing some reading on TIMERx, the correct version should be:
ISR(TIMER1_OVF_vect)