In late June, 2008 I perchased a 7 year old, 15.1h, paint, gelding. I've taken him to my lessons and to two shows (open shows) and he did great. However, when I try to jump him over some fences, he acts up. On the approach his pace isn't consistent. I try my hardest to keep him going straight and not stop but he zigzags slightly and slows down. I know that's because he is trying to judge the distance better and give himself more to time evaluate the fence but how can I stop this behavior? He also likes to stop right in front a the fence and then leap over it with his back rounded so I get tossed in the saddle. How can I stop this? PLEASE HELP!

I'd recommend that you teach your horse to jump fences first without a rider. If you have an enclosed ring and can set up a chute, you can set up ground poles first (3 - 4 ft. apart depending on his strides at the trot, which will most likely be closer to 4 ft.) Set up 3 ground poles first, and then go to 4 once he's confident and just trotting quietly over them. If you don't have an enclosed ring, you can do this on the lunge line (which may be better anyway, as it will give you more control to keep him trotting rather than taking off at the canter, etc). Once he's comfortable with four poles, then go to five. When he's comfortable with that, take away the next to last ground pole, and use it to set up a low cross rail with the last one (in other words, you'll have three ground pole, around four feet apart, and then a space of around 8' with a small cross rail). Continue trotting him through this set-up. Once he's comfortable and not hesitating, you can raise the cross rail a bit, etc. etc. The whole idea is that he'll really be teaching himself, and won't have to deal with any balance changes, etc. which would result with a rider being moved about by an odd take off or approach. Once he's comfortable with the single jump, you can add a second jump (in and out), but I wouldn't do a bounce yet unless it's extremely low, as the sight of two jumps in a row so close together can be a bit intimidating to such a horse. Be sure, also , to give him lots of praise when he makes an attempt, and even more when he gets it right. Remember, too, that the ground poles are set up for him to trot to the first jump, not canter (can and should canter to the second in an in and out though). Eventually, you can set things up for him to canter all the jumps, but teach him at the trot first, and keep him quiet after the jumps and, again, lots of praise.
Once you start riding him over jumps again, keep the same set up until he has a decent handle on things, plus you can always go back to it from time to time as needed.
If your horse continues to have problems with jumping, especially without a rider, you may want to have a vet check him out, as it may be a vision problem or some kind of soundness issue which is causing his hesitancy. If he does well without a rider and just has problems with a rider, then there may be a saddle fit issue or there may be something with your balance or position that's throwing him off.
Hope this helps.