Strange characters on alphanumeric LCD display showing randomly

Hey! hope someone can give me any idea, I'm having troubles with 5 alphanumeric displays, 3 of which are 16x2 and two of them are exactly the same, other two are same but 20x2, the only thing they all have in common is that where taken from some broken devices (phones, and something like an interface for don't know what) so there's a posibility, a bit remote but you know about murphy's law, on which they are broken.
They all work fine for a random amount of time (goes from 5 secs to about 190 secs, depends on the display) and then start filling the display with some characters, sometimes shows the desired data but where should be white spaces are the characters, or just works fine for a few seconds and again fails (let's say it works intermitently, or fails intermitently, depends on the point of view, haha)
I'm using Arduino Mega 2560 and a barebone Duemilanove with ATmega328, I've checked connections, bad contacts, and different sketches (LiquidCrystal "Hello World" and one I made for my project) Tomorrow i'm gonna get a brand new display to test if it's a problem of the displays or not... if isn't about the hardware, it's possible a bug or something on the liquidcrystal library?
ps: uploaded some pictures of the display failing, maybe gives some idea.

Any idea is welcome... thanks! and have a good day!!
Lucas