Actually, the situation you described is one of my favorites.
As soon as you see the con, go level (if your not already) for speed. When I see them diving, I start a gentle turn that I tighten slowly as they approach. Depending on the situation, I don't start the turn until they are around 2K out. As they get to 1k, I am giving them a full top view of the plane making them think they have an easy shot. The problem is, they are frequently too fast to pull enough lead and keep up with my turn. That's usually enough to thwart their attack, but getting them to miss is only half of my goal. The other half is to get a reversal.
To get the reversal, just as they get into gun range, go wings level, and pull up. Usually, they are pulling lead, and you are under their nose, and they don't see, or can't react to your change. The next step is to continue the roll, and watch them pass under you. Keep rolling and you are now behind them, offering you a snap shot. Often however, they are SOOO much faster than you, they blow by, and zoom climb. Your next thing is to start a low E turn away from the con. This will provide more distance, allowing you to do a slow climb, decreasing their advantage on their next pass. I keep doing this, changing directions (left/right) slowly bleeding their E. Frequently, after a few passes, they lose all their advantage, and now I am on their 6.
If you want a demonstration, PM me, and I will be glad to show you how I do it, and how I defend against it so I don't become a victim of my own move.