Memory management problems with ATTiny85
Follow the steps in this post to check your RAM usage:

Checking memory footprint in Arduino

My guess is that you have gone over as the RAM size of the ATTiny85 is 512 bytes. In your sketch, font will be stored in RAM. You want to store it in the FLASH program memory. Follow these instructions to do so:

http://www.arduino.cc/en/Reference/PROGMEM