I've tried everything to fix terrier chasing problems, or at least it feels that way. What has been working lately is taking it very slowly. We run our chasing terrier against an 8 second dog, if he does well rather than make it harder right away, we wait several weeks before introducing him to a 6 second dog. He is finally able to handle a 4.5 second dog 95% of the time.
I recently heard that it helps to use three boxes in a row and put the chasing dog in the middle. The theory is that the chasing dog won't be able to make a quick decision about who to chase, so he stays in his lane. The lanes were very close together. The dogs were all released at the same time and it seemed to work...or he didn't try to chase anyone. I think this drill could also be helpful to improve speed to the box.
THIS IS WHAT IT LOOKS LIKE