Training Ex-Barrel Horse as a Hunter

Have any of you ever dealt with training an ex-barrel horse as a hunter? I have an ex-barrel racer and sometimes he gets tense and starts rushing...We have tried upward and downward transitions, but these seem to make him even more tense...What method should I use to calm him down (the weirder thing is that he is often lazy, but on the days he is wired, his is wired!)? Thanks! Any advice is greatly appreciated!

My horse currently does both barrels and hunter. I love using grids for rushing as it makes the horse pay attention to his stride.

I've schooled quite a few horses/ponies that have this problem.
Towards the jumps the horse would start rushing and rushing and rushing till the point it is very unsafe for them to jump.
I use many transitions, half halts. I set up a grid and inbetween every jump I ask the horse to halt, then proceed. After the grid, I would get him to go straight to the end of the ring and halt at the fence, turn around a go through the grid again. Once he was able to halt, and proceed through the grid, we then would trot in and out of it, if they started to rush, then go back to halting, it took some time but works well.
Or another thing I do is ground poles between each jump, that really makes them slow down and think where their feet are going. Good luck!