Have changed throttle-position sensor and made a slight difference at first, but don't think the problem is that. Notice that if I kick it down into passing gear, it will clear up for a short while. Also it will back-fire occasionally, I have listened for "dieseling" or "pinging" but haven't noticed anything. Too, the problem is intermittant. I have noticed that it seems to run alright sometimes, until we have to brake, then it will start. Any Ideas what could be the problem?