Ever wonder if there's any markings inside yer violin not seen readily ? Any cracks that don't show outside ? Old repairs / patches ?
Several violins I've inspected / worked on had un-repaired cracks in both end blocks ...no wonder they wouldn't hold their tuning ...
Here's a pic of the lights I use ..note that the small light will not fit into the F hole but the big one does for thorough inspection ...mirrors come in different sizes as well...