Mine use to do that. It turns out that the +5v is not clean or strong enough. Even my TFT screen FEW days ago did the same thing. For the time being, i connected it to the cigarete lighter and it stoped. If u are geeting ur +5v from the PSU and ur psu i not connected directly to the battery, then my suspecion is that the alternator is disturbing or afftecting the current, i believe ppl refere to this as that sine wave thing. Either way, ur current is not clean. Also check the wire that supplies the +5v to the LCD.