When I practice sweeping, I put on the metronome at about 130 BPM and play the lick. I play each note at a metronome click. Is that (playing exactly to the click; fourth notes) how you are supposed to play? Because for some reason I can't practice with eighth notes or higher, because I mess up the timing and I don't feel that I'm gaining any dexterity using fourth notes.
Sweep picking is an interesting one to practice. Everyone has a sweet spot with sweep picking when they are starting, where the metronome is slow enough that they can hit each note properly but fast enough that their picking hand is moving smoothly in a controlled motion. Find this sweet spot (for me, it was eighth notes around 110BPM when I started) and make sure that you can hit each note perfectly and that your picking hand isn't plucking each note individually. Your picking hand should almost glide over the strings, and at too slow a speed you will be too tempted to just pick every string rather than sweeping across them and the exercise won't be helpful.
Start with an easy shape, like a C Major arpeggio, with one finger per fret (in order to help sync up your fretting fingers with your picking hand):
e|----------12-15p12--------| B|-------13----------13-----| G|----12----------------12--| D|-14-----------------------| A|--------------------------| E|--------------------------| D D D D U U U
Play this simple phrase over and over again, with each consecutive down stroke and up stroke flowing properly. You should not actively pluck at any of the strings. Play this at a speed which feels okay to you, where the metronome is slow enough that you can play the notes cleanly but fast enough that you are not feeling stilted. Once you can play this flawlessly, increase the metronome a bit and repeat the phrase loads of times, until you can consistently play it flawlessly at that speed, and then repeat until you are about 5-10BPM faster than you need to be. I always do this so that I can be confident that I have mastered the riff beyond where I need to.
Once you've nailed this, try incorporating more notes into the phrase, such are the
15th fret on the A string and the
10th fret on the A string, and play it in triplets (starting slow again). Or, you can try a different shape entirely.
My guess would be that you are just trying to practice a complex thing too quickly and are not coordinated enough to be able to confidently pull it off at the desired speed. Slow it down, take your time, do exercises like I suggested, and you'll get there with some practice. Guitarists like Paul Gilbert and Synyster Gates got to where they are today with routine and years of practice and playing. There's nothing wrong with attempting things out of your comfort zone, but just trying to attack it head-on will rarely help in the long run.
Also, remember that you shouldn't let more than one note be ringing at a time. If you end up in the situation where you cannot play the pattern you are attempting without letting the notes bleed, you should slow down a bit until each note is ringing out cleanly and there is not more than one note being played at a time.