Bex workbook Not responding

Hi all

When I run the workbook, sometimes it says critical error occured, somtimes I can run workbook successfully, and sometimes it keeps on executing for hours but not result and it will not responding.

It is not consistent in normal way, really not sure when it shows result set when it doesn't. Can any one please let me know is it something to do with Patch level?

Right now I am on 720 Final release, Patch level 0