I have been lurking on this one and trying to come up with a possible cause. When you said that your hand shielded the instrument, it implied to me that perhaps the internal shield may have become disconnected.

Ten years ago (I do not know the current practice), it was best practice to spray the interior of these moulded housings to provide shielding from external (and often internal!) sources. Your observation makes me concerned that perhaps the connection from this shielding to ground has been disturbed.

Just an idea...

Regards,

Michael