I've also never gotten a BJ to completion. The Mrs. gets me close but then tuckers out. The first woman to ever give me head had incredible stamina (probably sucked my dick for close to 20 minutes), but I never even got close with her.
I can get there with woman-on-top. However, I have noticed I lose some stiffness if I'm too passive, so I inevitably start thrusting into her, which isn't necessarily her "riding." If I just laid there and let her do all the work, who knows? I might have to try that soon just to see.