Ranco reading too high

My Ranco 111000 is reading 10 degrees too high. I just cooled my wort down to 68 degrees which my Ranco and dial thermometers both agreed on. After allowing to sit for 30 minutes and then transferring to carboy I unplugged the Ranco and plugged it back in; however, it was then reading the wort at 79 degrees. I checked with my dial therm and it was still reading at 68. I unplugged and plugged back in the Ranco, but still was reading 79. The temp probe did not get wet and I did not step on it so I don't think it got damaged. Any thoughts?

Think I answered my own question. I unplugged the Ranco for about 10 minutes and replugged it back in so it is now reading 70, that I can believe.