1989 cutlass ciera running rough

i picked up a 89 cutlass its got 83k on it so i figure perfect for the winter. its got the 3.3 liter v6 in it. my problem is when i drive the car for 5 min plus at 50 plus mph it tends to shutter a little bit when i let off the gas and the idle wants to stay at about 1000 rpm as aposed to the 800 before it hit the 50mph for a while. im just wonderin if it could be some sort of sensor that fouled or what else i should look into. the car doesnt stall out just idles higher and has that shudder. any thoughts? thank you for your time

Shutter is rough idle /misfire? If so look for a vacuum leaks.