Kinda sounds like liver. Sometimes it can take a while to do the job, and before he dies he'll be really struggling...after this kind of shot I've watched them run 100 yards, stop, walk another 30 yards looking really sick, stop, walk another 30 yards, stop...they usually don't go very far very fast and most of the ones I've seen were recovered within 200 yards, but they need to be left for at least a few hours. Sounds similar to what you're desribing. How did the hit look to you?