So, this invalid block thing was happening quite often until i discovered something: I was always running my CPU with an undervolt flag for better temperature under heavy loads.
Are you doing the undervolt though software (e.g. cpupower) or are you setting a hardware setting?
Maybe you're using a hardware undervolt and so CPU cycles are being thrown away as a result without Core realizing it's being slown down. I never had any problems with underclocking with cpupower, you should look into that (but its underclock is system-wide).