Interesting! Thanks for the explanation. Do you happen to know if the III sends more / larger SysEx data per second than the II (and Ultra)? Because with the Ultra I've never seen an issue like that.
All the more reason to use checksums! Well done to put them in your library!!
I don't know regarding the II. I think it's good that the III just sends a ton of tuner data, it is up to the receiver to deal with it - more powerful chips might do better. You can also increase the arduino input buffer size but I don't want to make the library rely on hacking the arduino core files.
Ah, yes, well... I WASN'T checking the checksums... AlbertA suggested I do so!