How can I improve my alternate picking string skipping? WHen i play slow its easy but when i add just a bit of speed i miss the notes.

Say i wanted to play the first solo off All allong the watchtower by Hendrix ... the ending is as follows

-9----9---------------9---------------------
--12b--12b-- 9-----9----12--9------------9---
----------------11b-------------12---11--9---
--------------------------------------------11
-----------------------------------------
------------------------------------------

played @ 120 bpm
How can i acheive this