Maybe I'm all wet on this but I don't think your nuts or studs are backing off - I think the relaxing of tension on the studs is from the gasket taking a set which relieves the expansive forces or rebound forces acting on the pan and fastener tension. Re-tightnig the nuts compresses the gasket slightly further, again restoring the expansive forces acting between the pan and the block. At some point after a couple cycles of this, the gasket is compressed to where it's density increases to a point the fasteners can't be tightened any further without exceeding torque standards and possibly distorting the pan, etc. I don't think double nutting your studs will do anything except make it a little more difficult to snug up the fasteners any further. I would think after 2 or 3 cycles of that you are probably near the point of diminishing returns.
That's my theory or understanding anyway - right or wrong.
