Algebric loop error for a PI controller feed back?

Hello,

I am trying to solve a model. The model consists of a feedback loop for a PI controller which controls the input variable. the feedback is taken from the integrator block.The model works fine in a steady state simulation. But when i run the simulation in transient state with varying other variable. It shows the following error

• Algebraic state in algebraic loop containing 'system' computed at time 720.000001 is Inf or NaN. There may be a singularity in the solution. If the model is correct, try reducing the step size (either by reducing the fixed step size or by tightening the error tolerances)

I have tried using memory and unit delay block in the loop. But the model wont work. I have attached the image of the loop error. Please guide me through this.