Eight note's, all downstrokes over two strings at 180bpm

Hi everybody
I’m trying to play eight note’s, all downstrokes over two strings at 180bpm as in the ‘Timewarp’ from ‘The Rocky Horror Show’:

… but even after a lot of practising an trying to relax the handi just dont get to the speed i need. Or cant keep it for more than a bar or two. If anyone had some helpful advice id really appreciate it.
Thx and have a nice one! :wink: