Sounds like a weak magazine spring, but there are plenty of other causes, as well.
What bullets are you using? Japanese military ammo was FMJ, so that could be an issue with feeding. Won't affect the bolt riding over a round in the mag, though. That is either a mag spring, follower, or possibly bolt itself issue.
it is also remotely possible someone altered the magazine feed "lips" which are part of the action rails on that gun. People have been known to file, or Dremel
them to "improve" feeding. Generally, it doesn't work, and they ruin the gun trying.
First thing I would look at is the magazine spring. The "Z" spring can be installed backwards or even upside down (and no, I don't know what the right way is off the top of my head) Shouldn't matter, but might.
More likely is just that the old spring isn't as strong as it once was, and the lack of proper support (tension) on the follower is why some rounds don't get into proper position for the bolt to pick them up and feed normally.
Nothing is as cheap as it once was but a replacement magazine spring shouldn't cost a ton. Get one and see if that cures the problem.