This is a fairly common problem in the lcd displays that were used in 1997-2000 cars. The best explanation I've heard is that an impurity found its way between the layers during the manufacturing process and that over time it causes the layers to delaminate. I think this was so common that it led to the led to Porsche introducing the dot matrix display clusters in the 2001 and later models.
When I installed a 996 cluster in my Boxster, I bought a damaged 996 cluster at a greatly reduced price. That cluster had a flaw on the clock/oil level lcd. I tried several methods to fix it including heating it. I eventually swapped the good board from my Boxster an reprogrammed it to make it work.
If you cannot live with this problem, you will need to find a replacement cluster.
Sorry to be the bearer of bad news, but I have been there. Good luck.