If it's starting to buck, it could be the ignition cutting out. I forget what it's called, but there's basically an electronic pickup in your distributor; if you have replaced it yet, do so. You may have to bring it to a shop because i think you have to press on and off the distributor gear to get it on correctly. My car got worse and worse and I replace 150 buck or more of electronics before I replaced that one 20 dollar part. many stangers end up just bringing it in to the shop, but definatly do that first. It basically just burns up after all the abuse a mustang goes through, believe me. It's worth a try anyways.
As for the coolant, it could easily be a head gasket if they're the original ones. That would cause it to buck every once in awhile as well. All it takes is a little pinhole and that water will gush through pretty good into the cylendar. you may want to put on new head with the gaskets though, lot better performance, lol.