User defined function crashes Matlab

I have encountered serious problems when running simulink models containing user defined function blocks, it crashes Matlab completely and will not re-boot. I have to uninstall and re-install to Matlab each time it crashes. I have similar problem when setting-up Real-time Windows target toolbox. Any suggestions?

[Kaustubha Govind]

That sounds inordinately severe. Is there a stack trace with the crash? Also, are the user-defined blocks part of some third-party library or did you create them? What types of blocks are they exactly (S-functions, Interpreted MATLAB Function, MATLAB Function block, etc.)