1987 Other Mercury Models Engine Light on - idling

The engine light will go on sometimes while I am sitting in traffic idling and will generally not go off until I get the car moving again. Sometimes the light will come on while I am driving on the freeway (55-65mph) but it will only stay on for about 10 seconds. My mechanic is having trouble diagnosing the problem. He has cleaned out the catalytic control valve (?) But the problem still exists. Before he did that the car was also conking out when I would get back from driving it (usually around 10 to 20 miles) and put it in park if I let it idle for a few seconds after putting the car in park. That has since stopped happening.