Aero Gas Turbine in cooldown trap


I am using TM2500 aero gas turbine with Woodward control.
During running conditions, the turbine is controlled by NPT speed control. No problem at running condition.
When the operator initiate stop command, the generator breaker is open and the turbine initiate cool down sequence.
As per logic the turbine should reduce speed to N25 references speed @6300RPM and the control move to high speed rotor control. The high speed shaft should rotate @ 6300 rpm for 5 minutes. Then Fuel chopper signal should chop the fuel.

unfortunately the 5 minutes timer is resetting after 120 sec or some time 240 sec I.e. different time. This making the cooldown cycle in trap as the timer is never completed 5 minutes.
The time can be reset in only two condition. 1. If the turbine is not in minimum speed reference .

2. If the control is not NGG control.

We found that some time the Turbine control moving from NGG control.
Any one can advice why this is happening or how we can solve the problem.
Thanks in advance