I also find that if I modify the clock frequency in the "boards.txt" file to
uno.build.f_cpu=1000000
I get inconsistent device signatures:
trial 1:
avrdude: Device signature = 0x0000f0
avrdude: Expected signature for ATMEGA328P is 1E 95 0F
Double check chip, or use -F to override this check.
trial 2:
avrdude: Device signature = 0x000000
avrdude: Yikes! Invalid device signature.
Double check connections and try again, or use -F to override
this check.