I had the same problem just 3 days ago. The day before I had a reading of 8.18 and all of the sudden I was down to 7.78.....I freaked! I re-calibrated 2 times and it never got better. Mine has replacable sensor heads, but it is fairly new and I didn't think that it could be bad that fsat, plus it will tell me when the head has to be replaced. So I did a quick water change thinking that it might raise the pH a bit, but no go. I took a sample to the LFS
and he got a slightly higher reading than I did, but not too much.
I bought some "Reef Buffer" from Seachem and put it in there, and 15 minutes later I had a pH of 8.25.....and it is holding steady. It worked for me, you may want to try it out.
125 Gal-35 Gal Wet/Dry-Tetra-Tec PF500 HOB
(3) AuquaClear 301 Powerheads-600 GPH
Automatic Tempature Controller
5-6 Inch Sand Bed-200+ lbs of LR