Audi A4 LCD screen faulty

I have an S reg Audi A4 (98) with 163,000 miles on the clock. The LCD screen was playing up for the past year. It would have a crystal clear readout one minute and then it would either go out completely or be very hazy and unreadable the next. I have tried replacing the unit with a second hand one that worked perfectly well but has the same effect now its fitted! Any suggestions on how to cure this would be greatly received.