Belle would get really aggressive around time for her heat cycle before we had her spayed, and she's not as bad now, so for me, the spay/neuter theory seems to be working. Another thing with her that we noticed (actually the woman running the boarding kennel pointed it out to us the only time we boarded them) was that when she feels very nervous, she is prone to lash out...in the case of the boarding kennel it was at Maggie, our other dog and she had to separate them into two separate pens. The woman told us that as soon as Belle realized we were leaving her there at this new, strange place, she immediately turned on Maggie. It wasn't so much an aggressive behavior, but an overly defensive behavior (she didn't have anything to 'guard' this time and nothing to be jealous of)...at least that was her opinion, and having seen it some more, I agree. When she is nervous or anticipating something/excited, she wants her space.

Does your JRT associate new people at the door with something bad (i.e. have people stepped on him by accident or something), or does the picking up maybe freak him out? Belle loves to be picked up (at 45 pounds, she can get heavy, lol, but her favorite is to jump up and be caught and held), but Maggie hates it.

Try to get him to associate good things with the situations he fears, and maybe the fear will go away and so will the defensive/aggressive (whatever!) behavior.