are you using ABS or PLA?
ive had PLA melt into a blob and harden outside the melt zone in the throat of the hotend. causes what you describe. only way to fix it is to take the hot end out and carefully try to pull the blob out. helps to hold the hotend in a small hobby vice with metal jaws, while still hooked to the printer, and turned on to soften the deposit. alternatively you can use a plumbers torch to heat the hotend and dislodge the clog, and if its ABS, you can skip all that and just stick the whole thing in a jar of acetone for a day or so, it'll be shiny and new when you get it out.
but you say that its an intermittent problem, like it works, then jams, then works again. that sounds like it could be heating element related or thermistor related. perhaps a solder joint has worked its way loose. this can be hard to diagnose without electronics experience. but the basic idea is to find where those wires enter the mainboard, and when it has the problem again, put a multimeter on the contacts set for continuity or resistance (depending on your unit) and see if its any different than when its working normally. you have to know what your doing for this because its possible to slip and short out your mainboard, causing a much more costly and time wasting repair. if you purchased a spare hotend with your printer, you can try just swapping it out, doing a PID autotune and rolling with it, see if the problem persists. the idea is to eliminate variables till you find the problem.
