Hi Bob,
Although I am not that familiar with your camcorder, your question hints at the problem. You say the Sennheiser has a stereo mini plug. This is not a stereo output but rather a 3 pole mini jack. I don't know if this output is a balanced or unbalanced, but the Sony certainly has an unbalanced input. The mic input to the camera also carries power, to power the supplied mic.
The mic that came with the camera is most likely to be a 2 pole minijack, is that correct?
Are you using the camera switched to mic or line input?
If the input to camera requires only a 2 pole jack that perhaps you could try a converter from 3 to 2 poles to see if that gets rid of the issue.
You might also examine the tip of the minijack, they are not always precisely the same in dimensions, check it against the one supplied with the camera.