For fast Punk songs, I use a pick, just like the original bassist. But there's this scratching /scraping sound when I play. Does it have to do with the way I'm holding the bass (while standing). because its fine when I'm playing while sitting.
I think the issue is the angle of the plectrum and the angle of attack. Look at your picking hand as you play sitting down and standing up, see if you see a difference.
Make sure the pick is parallel to the string.
You could try using a rubber pick or a felt pick. They work pretty well, though the felt picks don't last very long.