Check that the fuses are correctly set to work at 8 MHz. Fuses should be -U lfuse:w:0xe2:m -U hfuse:w:0xdf:m -U efuse:w:0x01:m
See issue #23 and try to omit the -B 1
switch in the makefile.
See issue #21 and help us fixing this in the firmware.
See issue #22 and help us fixing this in the firmware.