I'm actually having the exact same issue -- acatac I think we're twins here. I started leveling out in the upper 160s on a variety of PTs, most recent and then the oldest (7-14), then randomly kept going up and down between 177 (high) and 168 (low) on 14-20 (I'm up to PT 20, 174).
I know that some people are saying the more recent tests are easier, but the curve is "harder" so that could account for the sudden drop -- perhaps you were scoring in the mid-170s on the older tests, but dropped with the stricter scale? Do you notice your score changing but your number of incorrect answers staying constant? Any particular sections seeming to shift in right/wrong answers?
I found that my jumps reflect more scale shifts than actual number of incorrect answers changing. If that's not the case for you, maybe it's just burnout. I took a couple weeks off after I started walling at 169, and when I came back refreshed, started hitting mostly above 170 again.