I've had the same problem with my pair of wildies. Thistle (neutered boy) kept bullying Freddy (unspayed girl)...he kept chasing her around and biting her, but the next minute would be grooming her! :roll:
It actually got to the point where I split them up, but Thistle bullied his new partner even more, and Freddy actually became very dominant and bullied her new partner too.
I put Thistle and Freddy back together in the end, and they got on much better than during their previous relationship...so I think they muct have missed each other :lol: Thistle still sometimes chases Freddy, but not as much as before.
TBH I think wildies can be quite extreme and need a heirachy, whereas domestic buns seem happier to just rub along together.