Following the work I did in alog 40109 , this morning I tried a couple of other things. I put the ISI in damped state, and turned off the suspension damping loops, this didn't change anything. After, that, I restored the suspensions, recentered the T240s (with the buttons on the BIO screen) then drove to the end station and power cycled the 3 T240 chassis for the ISI (I left the box for the ground sensor alone). This seems to have fixed the 85mhz line in Z3. Looking at the coherence between the 3 sensors seems to be the easiest way to see if the line is still there, see attached plot. Red (Z2-Z1) and blue(Z3-Z2) are a measurement taken after the chassis power cycle, green (Z2-Z1) and brown (Z3-Z2) are from before. The dip at 85mhz in the brown line (caused by, I assume, the excess noise in Z3) is gone in the blue. I'll monitor, but I hope to never see this again...