I haven't tried to run your code, but I can tell you this:
When your uncomment the line which moves to the next record, what is happening is that Delphi is calling OnCalc again automatically as soon as the record changes, and you are probably trashing the stack when the call goes into an endless loop because each call to OnCalc moves the record which send it back to OnCalc.......... OnCalc is best used to just update the current record (or other data dependent on the current record). It is a very useful tool (I sure with C#.NET had it) but it can't be used for everything.

I'm not quite sure what it is you're actually trying to accomplish but it looks like you are using the wrong event.