Internal error with symbolic engine

I have written a code for neural networks. It includes symbolic function. The code runs fine for a couple hundred iterations, but after that gives this error:

"Warning: While saving an object of class 'sym': Internal error with symbolic engine. Quit and restart matlab."

Any idea how to get around this?

[Walter Roberson]

I suspect that the symbolic expression became so complicated that you are running out of memory while it is trying to save the expression.

Possibly adding vpa() calls might help.