Unfortunately I can only connect it to my netbook when I'm arround (it's at my aunt's place).
Also, serial monitor displays only gibberish, since I'm using pin 1 to drive my LCD, at 19200 baud (incompatible with serial monitor?).
So I can only display debug messages on the LCD. Like the amount of free RAM you suggested, still around 1485 bytes after about an hour.
The strange thing is that it only happens after 4 to 6 hours. I really don't understand why it never happens sooner? I did notice earlier, now that I divided my code over a few functions, that loop takes longer to complete, causing timevalue to miss one of the LCD screens (the fifth one, who shows the time the incubator has been online). So I had to increase the time interval each screen is displayed from 8 to 12 seconds.
Thanks again for all the help,