[distant_haze]

Hi, I've been playing guitar for about a year now and recently I've been detaching myself from simply learning song after song and have been concentrating on improvisation. I know a few scales here and there, all the major modes, pentatonic and the harmonic minor scale. Yet, I'm having trouble incorporating my knowledge of these scales into my soling. Most of the time that I do improvise, I do so using primarily the pentatonic, but most of the time I play what I hear, through which I do generate a few catchy melodies but as soon as I try a phrase in a very high speed it sound horrible!
I'm primarily a heavy/death metal and rock guitarist so speed obviously matter. What I want to know is how I can use the major modes and what not to improvise in a decent speed. And note it's not my technical ability that fails me, I simply loose track of what I play at higher speeds.
Plus, my knowledge of modal theory is seriously flawed. As I said, most of the time, I improvise simply out of the blue without any knowledge of scales or theory.It would be really helpful if someone could point out what scales or modes I could use for different chords and how to come up with speedy riffs spontaneously.