there are a lot of things that can throw you off... usually caused by breathing problems or the dreaded post nasal drip of chronic sinus issues. gakk!

since I have what I refer to as dyslexic fingers, I don't play in public.... but I do love to sing, including trilling along with symphonic pieces lol

I sing where I am comfortable and don't have to strain for notes. If I'm with intrumentalists, then I ask them to pick a key where they are comfortable and start off the song so I can pick it up when they repeat the first line. this is in informal settings - jams/sessions

for a performance, I make sure I memorize my "position" for starting the song... every note you sing has a slightly different stance, muscle position to control the air you release over the vocal chords... a form of muscle memory that you learn in addition to breath control. practice that to maintain consistency.

since I'm hearing the tune in my mind... not depending on an instrument to give me the next note... I can keep on pitch because I know how to sing each note - I hope that made sense

and I fully agree with Marje... especially about instrument temperment