I remember racing a cougar I hit like 117 or so with o/d off and though oh crap I forgot to turn o/d back on after 80mph or so, I flicked O/D back on and boom my speed drops to 100mph and goes nowhere. Still haven't figured it out but heres a rough explanation I came up with. Your straining out all you can of our third gear to get up to 115 so when you turn on o/d it shifts into 4th thus dropping the RPM's but since our cars aren't powerful enough that high we can't get any enough power out of 4th gear at that point to hold the same speed, thus our speed drops. That's just an idea I came up with but I'm not positive so don't quote me on that.