One of our male dogs attacks the other

We have two male dogs (and two females as well), a boxer mix and a lab mix. We've had the boxer for 4 yrs and the lab for 1. The lab showed up on our doorstep, and we adopted the boxer as a puppy. The lab is very well behaved 95% of the time, but on four occasions during the past year, the lab has bitten the boxer, two of the times for seemingly no reason. The other two were when there was food or a toy involved. The boxer is my wife's baby, and I am very fond of the lab. My wife wants to get rid of the lab so that the boxer doesn't have to walk around on eggshells. Plus she was the one who broke them up twice and got bitten herself during the process (once by the boxer and once by the lab).

Also - the lab is protective of our yard and did bite one of our friends when he came in the backyard. I hate to get rid of him, but we obviously also can't have him biting, even if only on rare occasions.

1. Are they both neutered? Are your females both spayed?
2. Do you do any type of regimented training with them? i.e. Nothing In Life Is Free, a.k.a. NILIF?
3. Do they know "wait", "give", and "leave it" reliably? If not, you need to teach them these commands ASAP and make sure they perform them reliably, in all conditions at all times of the day.
4. How is their basic obedience? And exercise?

Until then, you have to monitor resources. Because the 2 fights that you noticed were over resources, I'm willing to bet the other 2 fights were over an intangible resource (i.e. human attention or space). Here are some sample rules for a multi-dog household:

- Food is only fed during meals. Meals are fed in each dog's individual crate. The only acceptable time to give food outside of meal times is during active training.
- Toys are only given under supervision.
- Dogs must earn human attention (see NILIF)
- Dogs have their individual beds/space. If other dog intrudes, you step in and body block the offender.

You just have to implement structure and the scuffles will cease in time :) Good luck! Believe me, I know how trying it can be!