I'm troubleshooting an extruder heating problem. The extruder consistently preheats to 230C and starts printing at 225C (with PLA filament), but once the raft and model start to print it looks like the heater has shut down. The reported temperature starts to drop until after 3-5 minutes it gets below 180C and the printing stops with Heating Failure #3.

Is this more indicative of a bad thermocouple or a bad heating block? I guess I have spare parts from another R2, so I can start replacing both of them. Which should I try first?

I don't understand why it heats up nicely before the printer starts to move. Could it be a loose wire (inside the thermocouple itself or at the heating block)?