Why does the speedometer quit working and the check engine light come on and want to stull from time to time in a 1999 Chrysler Concorde?

It would help to know what model and year the vehicle is, but the first thing that occurs to me is a poor electrical connection, especially grounds. Trace the ground cable from the battery and insure that all connections to the body and engine are clean and secure. On most cars that I work on this amounts to four or five connections in the engine compartment. Also anytime the check engine light is coming on, have it checked for codes.