This doesn’t sound like a faulty unit to me; more likely a setup/processing issue, especially since your 4K discs are playing fine.
Standard Blu-rays are 1080p/24, so the player (or your TV) has to upscale them to 4K. That’s usually where these little “judder” issues creep in. A couple of things worth checking before you give up on it:
- Make sure 24p output is enabled on the player (sometimes called “24Hz” or “Cinema output”)
- Check what your TV is actually receiving — ideally it should switch to 24Hz for Blu-rays
- Try setting the player to output 1080p for Blu-rays and let the TV handle the upscaling instead
- Turn off or reduce any motion smoothing / judder reduction settings on the TV (these can make things worse rather than better).