I think my issue is resolved. I had a hunch it was a power issue; that the attiny85 was getting voltage spikes or something causing it malfunction. I was bench testing using a power supply and using a battery, but both using relatively long wires. I decided to just put the driver in my headlamp and see what happens, and it seems to be working with no problems yet. I guess the extra inductance of the long wires led to voltage spikes?