DC motor speed abruptly increase

We have a 850Amps Siemens dc drive 6ra 70.Recently we are facing a problem with the drive and the problem is that the actual speed sometimes goes on increasing beyond the set value and after sometime it returns back to normal set value but the drive is not showing any fault or it is not tripping also. The speed reference is set through touch panel and the corresponding analog output from plc going to analog input of the drive (speed reference) remains constant during the problem. The dc drive is used to rotate a roller which in turn is rotating a high inertia mould and the speed feedback is received via tachogenerator to the drive. The tachogenerator feedback of the motor at the time of fault is consistent . One time we saw that the speed was set to 300rpm at the touch panel and the motor was also rotating at 300 rpm but suddenly the speed started to increase, it went up to 500rpm and then again it came back to 300rpm. All the optimization run were done properly according to the manual and the drive is responding to all the speed changes most of the time. It is not also the fact the speed always goes beyond the set value just when a new value is entered sometimes the speed increases than the set value after rotating at a constant speed for longtime. Can you please suggest what other factors may be the cause of the problem and what are the remedies for those factors? paras attche in test file ..help me !!