RE: Xl extruder stops
It seems to be related to the cable strain relief. Since the last recommended upgrade, where the strain reliefs were screwed onto the covers of the dwarf board (the 3D models are available for download on the Prusa website), I've only had one more problem. Upon inspection, the cable was slightly taut, so I suspect I hadn't pushed enough cable under the strain relief to hold the plug in the socket. I unscrewed the strain relief, pulled the cable through two millimeters further, and reassembled everything. Now the cable presses the plug firmly into the socket, and it's been working ever since (about two month). However, I suspect it's simply a design flaw by Prusa; they should have used self-locking connectors. But since my XL is a bit poorly designed anyway (I'm just saying internal electronics in the hot zone, which doesn't allow temperatures over 40°C inside the cover, no internal memory combined with the occasional USB stick disconnection problem, no single head calibration...), this is only consistent. Maybe it's better with the newer XLs. But, to be honest, if it works it's really great, apart from the 40° limitation. I just had, like others, a lot of trouble to come that far.
RE: Xl extruder stops
It appears that heat is a factor - at least on my printer. Over the past week I printed over 10 parts in PLA without any issues. Today I switched over to PETG and only made through just one part before this started happening. As before, power cycling the printer fixed the issue.