Intermittent Starting Issue?

Have a 99 Chevy venture. It has an intermittent problem where sometimes the engine will turn over but not crank. Then it will crank. My mechanic says it is a fuel issue and gave me a can of starting fluid. But my rudimentary tests indicate it does not get spark during those incidents. I wonder if it could be my crank position sensor failing. There are no engine lights, so diagnostics have been useless.