Brian,
You've got the spirit to be an avid RC hobbiest. I know how anxious all beginners are to get into the air and get on with it. Unfortunately, I tried the same approach as you suggest...take it little at a time and get the plane a little higher each flight. But that's not a good idea. Learned that the hard way. The beginners best friend is altitude. One of the hardest things to master is landing without incident. When you take off with these planes you're committed to flight. Touch and goes come with practice. So, if you go if alone just try driving the plane around on the ground. Sounds boring but it will get you the feel of a couple of the controls. Find an experienced flyer with a buddy box connection.
If the plane runs for 5 minutes without fault and then suddenly quits I doubt the nitro mixture is the cause. After it stops, check to see if there is any fuel left in the tank. If its empty, the 5 minutes at a rich fuel setting is proabbly all you will get with a 7 oz. tank. But that seems a bit short. Check to see of the clunk fuel line inside the tank is free to move and that it is positioned correctly. With an empty tank you should hear it rattle back and forth as you tip the plane upside down. That clunk line should be long enough so that the clunk weight does not contact the rear of the tank but will hang in the bottom in any position. Good Luck.