What I have observed in my own experience, is that, each of thoughts, mental images or mental talks, comes to awareness with an intention, meaning each image or burst of mental talk has its own intention and appears in awareness with that intention, and when I turn attention towards observing thoughts, often this intention of observing thoughts is stronger than the intention of the thoughts, so thoughts stop from arising, because their intention doesn't arise in favor of this stronger intention for observing them, and it feels like if I have to have those thoughts and at the same time observe them, I should inject some intention their, in other words, I should initiate the thoughts again with an intention to see those mental images or hear those mental talks, and maintain a balance in the strenght of intentions, to keep an intention to observe, and at the same time, let the intention of those thoughts to arise.
Also, I think there is nothing wrong with thoughts stopping when you observe them. You can enjoy that! I can assure you that there will be times, that no matter how hard you try, you won't be able to stop them. Working with a quieter mind can help to have some tools in those times.
Shinzen's Auto Think technique is a very good one to work with thoughts. and Also his See In and Hear In, but both of those could be practiced with Auto Think.
