Why is my 1989 Buick Regal keep dropping in speed when I drive it , I have changed the fuel pump and filter a couple months ago and it ran fine, now a couple months later its missing real bad, so I changed plugs and plug wires and air filter and put fuel injector in it and now when I drive it , it hits a certain speed and stays there or it drops down, it also goes really slow up hills. please let me know what anyone thinks might be wrong with it. Thank you