Show us a picture of the jam next time you have it happen. That will tell us which of several types of feed jams you are having. Jam with nose on ramp, jam from nose diving of the round, three-point jam, etc, etc. Get a couple of pictures for best info - one showing the nose of bullet and front half of case, and another of the rim of the case and whether it has slipped under the hook of the extractor.
Happens under what condition? First round out of a full mag, or the last one out? Both of these situations may point to a magazine problem, but different causes.
Randomly? Could be the extractor is not tuned to pick up the round real well. In fact, that would be my first guess, especially with a lower price range gun. The extractor is not just for ejecting a case - it controls a "controlled feed" of the round into the chamber.
What the #*@) does "dropping a round into the mag" mean? You mean just loading a single round into the mag? I hope so, because if you are placing a loose round on top of the mag follower (not into the mag itself) or into the chamber loosely and dropping the slide, you are putting a LOT of stress on the extractor making it snap over the rim.
As for mags, I would not trust an "unverified maker's" OEM mag from Taurus. I would trust a Wilson or Tripp or Checkmate. Not so much the McCormick without replacing the spring and follower. But show us how your failure to feed occurs before you run out and buy anything!
Your "ejection of cases" test shows your springs are about right.
good luck, GJ