I've heard numerous comeback players comment that they can play things fine when practicing at home, but can't when they get to a band (or whatever) rehearsal. I suspect most of it is precisely the lack of practicing pieces at a correct, steady tempo. It's common to practice the easy stuff up to tempo and slow down a bit on the difficult stuff, and be sloppy about taking breaths. That doesn't work in a group rehearsal with a good conductor, though, so all of a sudden you can't play what you thought you could. Using a metronome while practicing is a great way to simulate a real performance so you're not surprised/embarrassed when you get to rehearsal.