| I apologize if this seems like a snarky response, but once you establish good time with your scales/inversions, etc, at the slower tempos, do you increase the metronome speed?
I liken metronome practice for speed to be like lifting weights. I establish what tempo I can do an exercise cleanly at, then I bump up the metronome a bit to where the cleanliness of execution *juuuuust* starts to come apart. Then I bump the tempo a bit more to where the execution is just plain bad. Write those numbers down, because just like lifting weights, tracking where you've been is essential for measuring progress and will keep you from getting discouraged.
Now, warm up at the 'good' speed. Then practice at the 'just starts to come apart' speed, until it's clean. (this might take a day or four). Once it's clean, congratulations! You've established a new baseline (no pun intended) for 'clean speed at given tempo X'. Write that **** down, yo! Now with your new tempo in the 'clean' column, figure out the new 'just starts to come apart' and 'sounds like crap' tempos. Rinse and repeat until you're doing syncopated 16th-note triplets at 320 ;-)
HTH,
--Lee | 
