Some people will say "hey nice group if you throw out the flier" well you simply cannot. it is there and needs to be addressed. Noone wants a rifle that shoots good most of the time. and from your post its not a cold or clean bore flier, its a random happening. All the more reason to find it and eliminate it.
Assuming all is right in your bedding and scope mounting and this is indeed a true reflection of your rifles performance you need to do some tuning. obviously the rifle is able to put several shots into a nice group, there are a few things I would do. FIRST you need to find out what is causing the Flyer, barrel harmonics? velocity? fouling? I think we can rule out fouling because you said it happens inconsistently might be 1st,3rd, 5th shot etc.. Is he shooting the groups over a proven chronograph? if so what is the ES of the group? do the fliers have a significantly higher or lower velocity than the rest of the group? if so you need to look into eliminating that from the equation, which would include NOT crimping anymore, annealing case necks, maybe switch to bushing dies if you are using brass with consistent neck thickness. Maybe a different primer.
Next Maybe it is in your barrels harmonics, each powder has its own burn rate. that burn rate has an effect on the vibration of the barrel and where it is at in relation to point of aim when the bullet leaves the muzzle. It is possible that specific powder is not the ideal match to your rifle.
Do not over look shooting rest, Boss Hoss can teach you a great deal on this if you PM him and ask him the proper way to set up your rest. Is your barrel free floated or is there upward pressure from the forearm? maybe try slipping paper between the forearm and barrel creating progressively more and more upward force see what that does to your group.
Also shoot larger groups. see what happens with a 10 shot group over the chronograph, and do it several times the more data the better. 5 shots is relatively worthless data in this situation. you need to be able to spot a pattern. So the more you shoot the better. and when you change something only change one thing at a time that way you can use the process of elimination. More than likely this will be something very minor and you will see that flier disappear and your group tighten up. the box or 2 of bullets that it might take to find the problem is a worthwhile investment in your rifles accuracy and your confidence in that rifle