Rough engine on 1990 Ford Crown Victoria

After engine gets to normal operating temperature, I get rough engine running at about 30 mph. Roughness disappears at 40 mph and beyond. Already replaced cap and rotor and spark plug wires on driver's side of engine. I will replace remaining wires tomorrow. Plugs were replaced last October.

1 answer 2 comments
This sounds like either a vacuum leak or a sticking open EGR valve, which would smooth out when you reached about 40 mph. It may also be that the EGR valve is opening too much, too early. Try disconnecting it and then drive your car to see if there is a difference.
I disconnected the EGR position sensor. The roughness problem disappeared almost totally although the Engine Check light is (harmlessly?) on.

What would be the next thing to do?
i am having a kinda of similar problem.... what is the EGR valve and how do i disconnect it?