I had similar, the heat would creep up and make the filament upwards soft enough for it to make a big blob and a jam happens ...
Ended up heat insulating the hotend tip with heater element, reattaching the thermistor as close to the tip as possible (small slot in the nozzle). Used fiberglass matt and kapton to wrap it around many times. Now printing nylon @ 270c, and have a 80mm cooling fan cooling the printable objects and at same time hotend.
Quality rose A LOT and now you can touch the hotend while hot without burning your fingers and as the heater cartridge is not as much used, there is less heat creeping upwards as well etc.