By using your program I am not able to recreate this issue with either ATtiny2313, ATtiny23 or ATtiny25. They all behave they way they should regardless of the clock frequency or if the CKDIV8 fuse is enabled or not. And I see no jitter at all. I have attached a screen shot from my measurements.
Best Regards, Ivar Holand AVR General Support Atmel Technical Support Team