Intermittent JTAG with MaxV Device

I agree with FvM, you are getting success for short JTAG commands that really have no long data transfers (.e.g device ID and erase). The verify/blank check and program steps require lots of data traffic across the JTAG connection - failure of these with success in other non-data intensive commands points to signal integrity issue. You should scope the TCK signal for integrity during the program or blankcheck actions. Sometimes, the capacitance of the probe will clean the TCK signal and you will see these program actions passing when you try to measure them (Heisenberg at his best). Regardless, first step is probing TCK.