I hate to ask, but have you calibrated your PH meter lately?

The reason I ask is because I just about fried some of my girls in veg the other day - I noticed some spots starting, so I checked my PH and it read at 7.2 so I pulled out my mad science kit and got it back to 5.5 then I checked my clones and they were at 9... adjusted to 5.5 then tested my flowering cycle, it was at 8.5...... tested my ph meter and it was off by 3 points >.>

Had to go back and change everything again lol