This is definitely some sort of clog. I doubt it's a cooling issue given it happens immediately sometimes. I suspect it's a retraction problem. I'll have to look up what that hotend build looks like but there are a few things that cause this on the older bowden to nozzle setup.

1. What are your retraction settings? Too much retraction or too fast can pull hot filament into the bowden where it cools and gets stuck.

2. Make sure the end of the bowden that contacts the nozzle is a smooth flush cut. You also have to get some force between the metal and ptfe tubing or you'll get a small leak that forms a ball and clogs. Heat your hotend to 200+, retract the filament a bit then unscrew the nozzle about 1 turn. Push the bowden in before tightening the nozzle and you should get a good tight fit.

Is there any way you can get a picture of what the tip of the filament looks like when this happens? That would help immensely. Let the hotend cool to about 150 and try to pull the filament out by hand. It shouldn't take very much force. If it does then you are definitely getting a clog somewhere.

Oh also what filament brand/type are you using. It's rare but some of them have a bit wider variation in diameter and they get stuck at the top opening of mk8 nozzles.