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I'm not sure why, but it seems like a lot of players new to Freespace make this mistake time and time again and it ends up causing a lot of unnecessary frustration.
It's fairly simple really. If you match speeds with your target, continue to point your ship directly at your target, and are anywhere but directly behind your target, you are going to close distance. This is especially true if you are leading your target to shoot at them.
It's simple geometry, if your target is turning and you keep pointing straight at it you are flying more and more perpendicular to your target, and as this happens the number of degrees per second you need to turn in order to keep your target in your sights increases. You are also taking a much shorter path to your targets current position than the one your target took, so assuming your moving at the same speed you will get closer and closer to your target because you have a shorter distance to cover. Needless to say, this usually ends in either you overshooting or colliding with your target, or you end up so far ahead of your target's 3/9 line that the number of degrees per second you need to turn to keep the target in your sights exceeds what your ship can do.
There are a couple ways you can fix this.
First is the obvious one, which is to slow down, you can do this by hitting the decelerate key while matching speeds to keep your speed a bit under that of your target. This is probably the first thing everyone does when they start getting uncomfortably close to their targets. This works well, but since you often have to move significantly slower than your target there are a lot of situations where doing this can get you really shot up, especially on higher difficulty levels.
The second method is to take a less tight turn, or "lag pursue" your opponent, pointing your ship somewhat behind your target instead of directly at it. By doing this you can maintain your speed without overshooting your target. Obviously this temporarily prevents you from shooting at your target, but because it swings you around behind your target, it puts you in a postion for a better shot. This is excellent against faster but more fragile targets like those pesky shivan dragon fighters.
Anyway, I hope this saved some of you some potential headaches.