Pretty simple, every eighth note in the main part of the riff dips down a tone
(from 7 to 5, from E to D) I think - it sounds ok anyway! 
|-----------------------------------------------------------------|
|-----------------------------------------------------------------|
|-7-7-7-7-7-7-7-5-7-7-7-7-7-7-7-5-7-7-7-7-7-7-7-5-7-7-7-7-7-7-7-5-|
|-----------------------------------------------------------------|

|---------------------------------|
|---------------------------------|
|-----------------5-5-5-5-6-6-6-6-|
|-7-7-7-7-7-7-7-7-----------------|

Every now and then he cuts the main part in half, I think; have a listen yourself
and it's easy enough to know when you're wrong! I checked live vids and it looks
like this is where he plays it on the fretboard alright. He plays around a bit in
the live show as well. 

Emm, anyway... first tab, be kind  with the ratings!