I was about to say I had no idea how that could be the cause - but then I realized: The ADC clock would be way off. Maybe that could cause it.
Anyway, you need to burn bootloader whenever you first use a virgin AVR microcontroller, and whenever you want to change the clock speed or clock source.
Note that there isn't actually a bootloader in the case of most attiny's - but you still need to set the fuses for the selected clock speed (they're set to 1mhz when new) and clock source, and that's what burn bootloader does.