After a few more hours of sifting through forums, I found the answer in this very forum, in Bruce's post (#2): http://www.picbasic.co.uk/forum/show...2824#post42824

If you enable interrupts, then you need an interrupt handler. If you prefer just to monitor the
timer1 overflow flag, then disable global interrupts.
I'm not sure why this would be the case, but I'll work it out!