Could partly be because of the cross. Are you certain there is nothing bothering the dog physically? If that is out of the question I would also try running starts. If you want a fast moving dog start with them on the line. Some say to lead out but when I'm really looking for speed (ie: Steeplechase) I get right on the line with the dog hands around them getting them all rowled up before I take off. Dogs LOVE to chase. So be the item that they're going to chase. I try to keep quiet when I run but when they're going into tunnels and the chute I really encourage them on with a high pitched "woo woo woo woo" or "Go Go Go Go" something that really founds fun (make sure they're committed though) and take off like a flash yourself so they see your movement when they're coming out. I would break down your training and work on the speed. One Jump quickly than Two and a tunnel and play. Than two jumps, tunnel than back over the same two jumps. Break your training down and try to find the underlying problem. Have you moved forward quickly in training, is there something the dog doesn't understand? And at this point DON'T correct the dog if they go to do something wrong you continue on, at this point the dog can do nothing wrong!! Make sure that your having fun too!