Empisal dress maker 270d. is missing stitches. sews perfectly for a while then skip a few stitches. I have replaced the needle with a new one, and cleaned bobbin casing and checked tension. Now what?

Could be fabric not feeding evenly, heavy fabric with weak needle, erratic top thread tension (top thread snagging).

Here's how I test tension.

Needle up, presser foot up, and tension selector at 4. I make a slow steady long pull of top thread (zero tension w/ foot up). Thread should glide smoothly and evenly through the needle eye, with little hesitation, snagging or uneven feeding from the spool. Lower the foot down, make another slow steady pull of top thread. Again the thread should advance smoothly and evenly through the needle at the set tension level 4 (foot down). Note difference in tension at set tension 4 (foot down) and no tenion (foot up).

If I find no problem with top thread feed/tension, I then perform a like test of bobbin thread. Make a slow steady pull on bobbin thread. Again, thread should advance from the bobbin smoothly, evenly, and have tension somewhat like the top thread at level 4 (foot down).

Wrong size or type of needle, Poor quality thread, Feed dogs need a clean, Thread stuck in tension discs?