DMC200U Speed Monitoring fault

we have a DMC200U with a I530 Heidenhein control, which kept coming up speed sensor fault, the machine would run ok with linear moves and soon as the spindle came on the brake on the machine would come on with an alarm Speed Sensor inactive, we have had the spindle drive and the encoder changed but now the machine runs fine for about 3 hours and then bangs off with a fault Speed monitor inactive, can anybody throw any light or point us to someone who can solve this issue?