Dear Bert:
Thank you very much for your reply, but how can it be a software driver problem, I mean it can't be, because:
1- the 8051 is Ok, because the program runs as before(it has also a bit banged I2C driver which reads from a memory peripheral and the content of memory is shown on lcd, and it works as before, if the 8051 was broken then certainly I2C driver couldn't function, could it ?)

2- the driver used to work for more than a month without any problem. I'm sure after holidays when I buy new LCDs, the same driver works with them. of course If you really think the driver(software) can be faulty, I will try to debug it again.

3- the voltage now is completely stable 5.03 V,
if there was an over voltage it was for a relatively short period of time ( in matter of minutes utmost)

If you see I didn't get your meaning please explain.

one thing else that I noticed a few minutes ago, when the whole circuit is off -in the same area of two characters that I said are not shown at all( characters 56 &76)- dark lines are displayed randomly when I touch some parts of LCD, it seems that they became sensitive to static electricity(this happens only for these two characters). It seems as you said that LCD driver circuit(44780) has broken. I only want to know what can cause this situation so I prevent it in final product. you suggested extreme voltage & temp can be the cause but is it possible that voltage drop anything between 0-5v cause this ?

Thank you very much