I'm with Elwin on this one. This past week, my teacher told me to never play a song faster than I could while thinking 3 or 4 notes ahead, and knowing what I was going to do next.  That's at least 3-4 times slower than normal on a new song, and possibly as much as 20 times slower on a fast, difficult piece.

Part of what you're supposed to do with the thinking ahead is figuring out the best way to handle problem areas like the ones you're describing.  Is it better to start higher up, so that all of that phrase can be played on one string, or better to stay lower, because you're going to need your fingers there for the bit after that? If you play things slow enough, you can generally figure out what works best (for you). It's just frustrating, because the music doesn't sound anything like the music you're trying to play. Once you have the problems sorted out and it's just practicing it, the speed will pick up a bunch.