ADuC847 communication issue

We have been using the ADuC847 for several years. Lately we are getting communication error when trying to reset the controller. The response from the controller indicates its a baud rate error. We tried to replace the crystal or controller and that sometimes work and most of the times do not. We are getting about 10% failures on the production line. We would like to get to the root cause for this problem and would appreciate any help to solve this problem. We are using the ADuC847 52-pin package. In addition some controller would work initially at the manufactures and then later would start getting the same bad characters indicating communication error and does not allow to load any programs.