I agree. I think for the first few weeks Jack was settling in and was probably very eager to please as he was only just getting to know you, and how far he could take it with you. It does sound though, that now hes made himself at home, hes starting to consider himself in control when you should be!

The "pack leader" theory is very much debated on the forum and so I'm sure others will post conflicting advice, its your choice what you wish to carry out. Some people say that in order to become the pack leader you must send clear signals to the dog in order for him to know that you are boss and he is bottom of the rank. This, it is said, will make him much happier as he no longer has to feel the need for challenging you, and he will settle happily within the family knowing his position.

The actions to take in order to show him this include always eating before you feed him, making sure you go into new territory (a new room/outdoors) before him ie. never let him walk through the doorway in front of you... never give him any attention (not even eye contact) when you return home until he calms down, and always win more games than you lose (in regards to tug of war games for example). Also you could teach the leave command so that while playing, he must always give the toy up at your request. These things, apparently, will let him know that you are top dog, not him!

Many people say that the pack theory in todays domesticated dogs holds no relevance whatsoever and that the dogs know we are not dogs but we are humans... but we have had a lot of challenging behaviour lately from zach, he is approaching 6 months and becoming a nightmare, so I am trying these techniques for a while to see if we get anywhere x

ps. in regards to his mad sprinting sessions, I think all puppies do this, its getting rid of any excess energy they have. Are you exercising him enough? Even if you are I think he'd prob do that, zach has his five minutes of madness in the evening then flops!