This will be WAY to basic for you but for me it took me back to the basics and forced me to go down the list and set things accordingly. As you said, the settings on the 800 can mess up the jpegs and make things unrecoverable so maybe now is the time for me to switch to RAW. Looking at your images has inspired me some and now that I have repeatable results with my 3 best lenses and the TC I should probably switch to over to RAW.
My changes/validations were: ..Pic control to neutral w/settings to 0 for contr/sharpness/etc. ..ISO ctl ON ..Max ISO = 1600 ..High ISO normal ..ActiveD & HDR to OFF ..Minimum shutter 1/250.. but always keep it at 2X focal length ..Center weighted at .8 ..WB auto but always check ..VR off on tripod active handheld ..Adjust EXP CTL to the right on histograms ..Also being aware of when to use AF-c and AF-s had a big impact. With shallow DOF and body sway the AF-s gets me in trouble and for moving bugs, AF-s lets me lose focus. I always use center point focus with AF-s focus /release and AF-c release settings.
Like I said, I don't know what happened but when I reset and checked my settings my picture became consistantly better.
Dan (Nikon D800,V2,Sony HX400V,Lumix ZS40) "I don't read, I just look at pictures" - Andy Warhol