All of sudden, speed drops and car moves @ 10 MPH. We stop, and start again. Run fine. Happens almost once daily. Is it doing that because the throttle body is dirty or it needs to be replaced?

Hmm, it sounds like the vehicle is going into 'limp mode' which is designed to let you limp home, then it resets when you restart the vehicle. Code would be helpful, but throttle body unlikely to make it act the way it is doing. Multiple problems may throw vehicle into limp mode, including a severe misfire, or problem with throttle position sensor. Suggest diagnosis by competent technician. We will be happy to do that here for you. Please contact shop if you wish to schedule a diagnosis.

Well, I had a similar issue so I washed my mass airflow meter. Give it a shot too.