Give it a clean, just for fun, nothing to lose. Take some readings before, put in a ziplock filled with Isopropyl alcohol overnight. Leave to dry. Put back in take more readings and plot the two together to see the difference.
I even brushed it gently with a glass fibre brush. I have one of these, but its too big, so I take a few hairs out and make a small bundle which can fit the opening to reach the metal element.
My theory is, if its gunk/oxidation on the metal element surface that's causing low readings, you need to gently scrape it off. Should be possible since thats how electronics contacts/boards are cleaned, IPA and glass fibre brush.
http://www.expotools.com/acatalog/10...ush-70530.html
or maybe look for a better thinner brush. Ideally a small 90-degree toothbrush with glass fibres is needed.
I can chop off some fibres from my brush and send them to you for free if you want to have a go at making yourself a home made mini brush. Perhaps from some 2mm aluminium tubing or similar?