Hot end popping out

I have a Prusa i3 with a J head .4 mm tip 3mm filament . I have been printing happily for months. Until last night when the metal part of the hotend was pushed out of the threaded part. I cleaned it out and threaded it back on. This happened three times today. I double checked to make sure the calibration wasn't off on the extruder motor and all of the axis, they were all fine. I'm using pla and slicer with all default settings which has worked perfectly for me in the past. Also the hotend has popped out the same spot every single time, second layer 3/4 the way through second layer. So I don't think the extruder is over feeding and the x axis is moving and the correct length and I did clean it out so there was nothing in the hot end. Also it is never jammed, if I heat it up and push pla through it, it extrudes freely. Any help is appreciated .

When I used the stock jhead I had nothing but rabid nightmares. The Teflon tubing in the middle always melted and I could finish a print without extreme oozing or jamming.if I understand you correctly, your heater block is falling through the threads? If so, I had a similar issue. I heated up to 220 for ABS and the nozzle seemed to get longer, but the hotend actually melted through the threads and once it happened, it continued to do so no matter how hot. I would suggest getting a hot end where there is no plastic threads for this reason. I upgraded to a hot end on eBay for around 60$ that has been working really well for me. If you are interested I could send you a link.

You can try teflon plumber's tape to give the threads a little extra bite, but the first thing that you should print is an adapter for an all metal E3D hotend to get away from PEEK.....