I'm the same way. Any other exersise I can get a rep scheme simmilar to: (8, 8, 7, 7) but with curls it would be (8, 6, 5, give up). Even if I'm not going to failure with the curls.
I havn't curled in a while now though. That's why I keep chin-ups in my routine instead of pull-ups (for the most part anyway).