Output of simulink model goes unbounded when sample time is increased.How can I fix this?

When I increase the sample time of the simulation from 0.001s to 0.01s the output of my model goes unbounded.I have used a PID controller to control the model response and when I checked the output of the integrator in PID controller (I used separate blocks to create a PID controller) it goes NaN after 4s. I get satisfactory response when I use 0.001s as sample time.Why does this happen and how can I fix this?