Got Problem loading a dll repeatedly

Our Matlab script calls repeatedly a mexfile in which a dll representing a model keeps being loaded and unloaded. After about 1500 cycles of loading and unloading the dll cannot be loaded by the Matlab-process anymore(it only works again if Matlab is restarted). This error cannot be reproduced in a C++ enviromnent, where I could load and unload the same dll more than 1600 times without problems. What could be the cause of that behaviour and what can be done to avoid this?

[Geoff Hayes]

Are you observing an error at the point where the dll cannot be loaded? If so, what is it? Why must the code repeatedly load and unload the dll?