What surfaces are you running on? Pavement? Grass? What's the C-Rating of your packs? Are you running full throttle passes constantly? It might not be thermalling out, it might just be hitting the low voltage cutoff. It doesn't take much if you're driving it hard to pull the batteries down below LVC. Get yourself a couple cheap low voltage alarms and set them for 3.6v. They'll let you know when you're hitting your packs too hard and getting close to LVC. You'll be surprised how fast you can get there when you're driving it hard. The Nero is a big, heavy truck.