After a long fight against this bug, I think that I've finally understood what is happening. The compiler in newest Arduino IDE, is no longer keeping the 'timer0_overflow_count' variable, despite the "extern" statement, because it can't see the symbol inside the following asm code. while this worked fine in previous Arduino IDE versions.
详见链接 https://github.com/fabriceo/SCoop/issues/4
新的scoop库 https://github.com/soif/SCoop/tree/fix_scoop_compil