I have a 2000 Honda Accord. It runs smoothly until I get to about 30 MPH, then the RPM's shift down, but the engine still feels like it's in overdrive. As I go faster, I can feel the car shifting, but the engine still feels like it's overdrive with some noise of the engine sounding like it's not shifting.

OK, so what were the codes and in what order? Your problem sound electronic in nature, so because of so many sensors and switches on modern cars all of that is important to know, because they are usually wired in series or parallel (each sensor sends/receives info to/from the computer and the computer in-turn sends/receives signals to and from the other sensors,so when you have multiple codes, you need to start with the very first one, and work your way down the code list to the last one, but as you do the repair for each code, you have to clear the codes and drive the car a bit to reset certain parameters, then re- check to see if each repair has solved the problem. You do this for every code until it is fixed. Unless of course you own all the expensive diagnostic tools that the dealers do, and if you did, you wouldn't be posting here.