"But P7 has a different version of Python and it may have different exception handling, so I suspect this may be the cause of the crashing.  I hope."

Nope. Exception handling works perfectly well. There are not mutch known errors in Python that could bring your script in trouble. But, it is a bad idea to have a try/exception that is very often executed. For any entry into this block Python has to generate debugging code etc. Python is an interpreter, not a compiler.

Among other troubles, WorldVertex coords can't be used as a point of reference to set deltas within the callback loop.  Trying this causes bizarre fluctuations of the delta position.  Weird.

Perhaps this is one of this nice things I described above. Changing something inside a callback function shouldn't generate another callback! Perhaps this is the case in your script.

Maybe I find the time to look a bit deeper inside your script tomorrow, surely monday.

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