Question: My 6 year-old gelding does a good flat walk but when asked to increase speed by either kissing or gently touching his sides, he raises his head, hollows his back and
goes straight into a fast pace. He fox trotted when I first got him although he was high strung even then. But now he just goes straight to a pace.

After unsuccessfully trying to get his fox trot back by trying to get him to tuck his nose with the curb bit, I have taken him off the curb and started using a full-cheek linked
snaffle. He gives to the bit well at the walk, but his energy level increases dramatically when whenever he is asked to increase speed. I have been riding him a 100 x 100
paddock and asking him to give to the bit while he is pacing and he seems to slow down into a headshaking fast walk, but I still can't get him to fox trot in the paddock, let alone
on the trail.

We rode with some friends with fox trotters a couple of weeks ago, and he fox trotted great on deep sand trails while he was in a line with fox trotters in front of him. But when he got into the lead, he almost leaped into his high headed, hollow backed pace and tried to take off. When he does that, I make him do hips around or do small circles before we move off again. But I'm really stumped about what to do to try to find his fox trot on hard ground or when he is in the lead.

Thaks for your help.

Collection, collection, collection.   You need to teach your horse to give to the bit and collect whenever you ask.  You might want to spend more time in the sand where he tires out more quickly and gaits better.  Until he learns to collect better, you might want to avoid the harder ground and fast trail rides.

In your paddock area, do a lot of stops & starts to break up his routine of hollowing out and going too fast.  Whenever you stop, back up and then go forward.  This should help him flex at the poll and learn to give a little better to you.

Laura