I have a '99 S80 T6 with approx 140,000 miles. This morning the "Check
Engine" light went on and the message window started displaying the
"Emission System Service Required" message (it's not the gas cap). Here are
the current symptoms:

Very rough idle
At very low speed 5-10mph major jerking
Ride becomes smoother with higher speeds
At 50-80 mph the "Check Engine" light flashes about once per second instead
of steady on, and the ride is fairly smooth.

Any ideas/clues would be appreciated! Thanks, in advance.

Alex L.