Hi,
If I read this correctly you have an interrupt frequency of 61.035Hz (prescaler 1:64, 256*64=16384, 1/0.016384=61.035Hz) when you ideally should have 61Hz - that's 0.057% off and as Anand says 2 seconds over one hour is 0.055%.

I currently have no idea why it starts running correct when you remove the DS1820 code - are you sure that's the case?