I also had this problem. I can play quite well slowly, but when I'm readying myself to play fast passages, I discovered that my picking hand tenses involuntarily so that my picking becomes inconsistent & easily tired. Sounds damn ugly too.
I solved this by using that 1234 excercise above, extend it to high frets, and using a metronome. Try setting the metronome to click on 16th note (4 clicks per beat), and start at around 50-ish bpm. Pick each note on the click cleanly and force your picking hand to relax at all times without dropping the pick. Increase gradually the tempo by 5 bpm each time to target tempo of 150-ish bpm. Every 20 bpm or so increase in tempo, record yourself to make sure you had each note picked cleanly. When you spot a mistake no matter how small, lower the speed & try again. DON'T use any distortion. It'll cover up any mistake you make. Use a slight overdrive if the clean sound is driving you nuts to give some color to the sound. The goal is to be heard picking each 4 notes even using an acoustic.
This may sound boring. Believe me, it's damn boring. But this excercise is required of you want any skill in playing. I managed to reach 150 bpm in about a week, one hour of practice each day. Now I can play at 150 bpm even immediately after getting out of bed in the morning. I still keep increasing the tempo, now 170-ish bpm is my limit so far before I got confused since the metronome clicks are so close together they start to blur.
Good luck :)
