When I'm playing by myself, I often have trouble coming in smoothly and without a hesitation. I sort of "lock up" in the back of my throat and my tongue. Whenever I use a metronome though, or have someone conduct me, or even just snap my fingers, I can come in fine and with no trouble. Also, whenever I play in an ensemble I generally have no trouble coming in, unless it's a very free, exposed part that not's quite in rhythm. Has anyone else ever had a problem like this? What would you suggest to overcome this problem?