Thermal runaway with specific prints
Thermal runaway protection (see What is Thermal Runaway Protection?) is triggered when the scheduled voltage to the heater element does not result in a specified increase in temperature within a specified timeframe.

The exit of hot filament from the nozzle and the loss of heat of the heater block and the conduction heat loss through the heat break to the cold end need to be supplied by the heater element. When the filament melting temperature is high, heat losses are relatively high as well, this means that the software needs to compensate by scheduling the heater element more quickly/activated sooner (there is some heat capacity in the heater block) and longer. If it is unable to provide enough heat than is being used, the temperature cannot be maintained and the printer notices that scheduling power doesn't result in a temperature rise. This will trigger the thermal runaway protection in the firmware.

This can imply that too much heat leaves the nozzle in the form of hot filament, and as such, your printing speed may be too high (or the printing temperature too high). 

Note that you are using a 0.8&nbsp;mm nozzle diameter, this means a flow increase of $ \frac{0.8^2}{0.4^2}=4 $ with respect to a "standard" 0.4&nbsp;mm nozzle! A slower print speed is therefore advised anyways.

To solve this, you can either:


Insulate the heater block, e.g. with a silicone sock
Install a different type of heat block, e.g. a E3D Volcano which has a larger heat input zone
Print slower
Print at a lower temperature
Change the firmware settings of your TRP (should be done with care!)


To explain the latter option, from the Configuration.h you can read that if you encounter these problems, you can tune this in the Configuration_adv.h

Excerpt from Configuration.h:


 * If you get "Thermal Runaway" or "Heating failed" errors the
 * details can be tuned in Configuration_adv.h


Excerpt from Configuration_adv.h:


 * If you get false positives for "Thermal Runaway", increase
 * THERMAL_PROTECTION_HYSTERESIS and/or THERMAL_PROTECTION_PERIOD


So you need to change these constants:

#define THERMAL_PROTECTION_PERIOD 40        // Seconds
#define THERMAL_PROTECTION_HYSTERESIS 4     // Degrees Celsius


The most effective solution for your problem will be limiting print speed and experiment with printing at slightly lower temperatures.