I am having a very similar problem on my 2012 MIEV.
Basically, when driving at highway speeds above 60 MPH the turtle shows up during acceleration. Using CANION, I have determined that turtle turns on when pack voltage drops to around 270 volts, which I understand is ~3 volts per cell (assuming 88 cells - is that how many this pack has?). While turtle is illuminated the car will regulate the power as low as it needs to in order to keep the pack voltage above ~270V.
Also, there does not seem to be a bad cell per se. When at rest, the cell voltage difference is 0.010 Volts. When under load with the turtle illuminated, the difference is closer to 0.4 Volt, with about 10 different cells at the low end and maybe 5 at the high end. Pack temperature seems fine at under 10 degrees C. I'm in Columbus Ohio, so outdoor temps were around 25*F when I got the data a couple of days ago. Last year this problem only occurred when outdoor temps were around 0*F. Now I can get it to happen up to around 35*F.
I'm starting to wonder if this is a pack age problem causing all the cells to have higher internal resistance than when they were new.