There are lesser quality lcd's but the chances of getting a broken one are rare but not impossible, as your getting the blocks at switch on i'd say yours is working and if your using usb for power then the 5-10 sec you mention would fit in with the blocks appearing and disappearing while the usb port goes through the device ID proceadure, please post your code and check your wiring, especially RS & E pins 4 & 6, if they are the wrong way round it will give strange behaviour.
At the very least disconnect the whole lcd so there is no Arduino influence and just connect Power & contrast to be sure thats working first, then control lines and data