Hi @marobinson71
Welcome to the Sonos Community!
Sorry to hear of the issue you are having with playing your turntable on your Era 300 speakers when using Line-In.
My suspicion is that either the cable is faulty (seeing as things work well with wireless options), or that the Line-In feed needs a little more buffering.
I recommend you first try increasing the Audio Compression: Settings icon » General Settings » Audio Compression » Compressed. This will reduce the quality of the audio slightly, but should make playback be more reliable.
If that did not help, please reverse the change made. One way to test things would be to connect a different input device (like a CD Player or a phone with a headphones socket) to the Line-In adaptor to see if playback improves. If that plays without issue, then I would suspect the integrity of the turntable’s audio cable.
I hope this helps.