If you want my advice, try to play most of the fingering patterns you can creat for any riff (only logical ones, don't make impossible patterns) and practice them all. This will increase your finger strength significantly (I've tried this method before and I liked the results :) ) . Then you might feel comfortable and faster with one or two patterns , so you will find the best that works for you and stick with it.


And don't forget the Historic Advice :
Always begin practicing very slow & raise the tempo gradualy.