Maybe A/C clutch is making the noise. I'd pull the belt and spin all the pulleys too.

I just doubt it's valve train because it would make the noise all the time and it wouldnt make a "kerchunk" noise if the lifter was sticking, more like a tick and just a noticable misfire. It shouldn't just go away for a little bit and out of no where come back, run like poo and up and leave again.