I've been having a lot of problems with my 2003 Nissan Xterra recently. At first when I started having problems it was because it was running really rough and the check engine light was on we hooked up the computer and it read as knock sensor and multiple cylinder misfire. So we replaced the spark plugs, spark plug wires, knock sensor and intake manifold gaskets and torqued them to the right specifications and also replaced the fuel filter(after starting to replace things on the vehicle we discovered that all of the parts were original that we were replacing and it has 202,000 miles on it). After all of this the vehicle ran great for about 12 hours then all of a sudden the gears wouldn't shift. After replacing everything else and it was all original we decided it may just be the transmission fluid we checked it and it was really old and bad. We changed it and the filter was terrible. We start it up and it sounds amazing, I try and drive it and it will only shift 2 gears(automatic) and it is a really hard shift and the first shift will throw it forward really fast and then go back up. What could be causing this?