Hi Mcnobby,
your ATTINY project is great!
I've seen the video and your sourcecode.
The problem can be a little jitter in the execution of your interrupt function (1clock or more of delay that depends on the instruction that is in execution when the interrupt happens) . I have got a similar problem but in my case the signal was more disaligned (1pixel or more).
I've saw this video by Charles CNLOHR before i realized the problem.
Otherwise can be a clock problem.. Have you looked the signal with an oscilloscope? It's ok?