TAZ 6 w/dual extruder temperature problem

I’ve had a TAZ 6 for about 3 months and have run several prints, added the dual extruder (V2.1) about 2 months ago. After a lot of tweaking and tuning, I was getting good quality prints using various filaments (PLA, ABS, TPU, PVA, PC).

Just this week, I was printing with Polycarbonate (PC) at 260 C, and the print stopped after about 5 minutes, producing a “Thermal Runaway” error. I re-started the print and watched the temperatures closely - for some reason, the temperature starts dropping very quickly after the first couple of layers. I’ve never seen this behaviour before.

During the wiping/leveling/heating steps, the hot end easily makes it to 260C and holds just fine, but shortly after it starts the print is when I can see the temperature drop, maybe one degree every few seconds. I don’t understand why it’s not able to hold temperature, especially since I’ve printed with no problems with this material at this temperature (and even a bit higher when I was profiling). The temperature drop may coincide with when the main fans start (I think they’re set to stay off until layer 3), but again, that was not a problem previously.

I did have the extruder assembly off the machine before this started to happen, as I’m planning to put in a fan and shroud around the heat sink to prevent heat creep in PLA prints, but I’ve re-assembled and double/triple checked to make sure it’s all back to OEM state.

Any suggestions on what to look for would be appreciated!