Time to rebuild?

Yesterday I got out on the highway and did the Italian tuneup. I didn't notice any smoke. This was after adding 2 cans of BG 44K to a full tank of gas, and driving until the tank was about 1/3 full. After I got home I hooked up a vacuum gauge to the intake manifold vacuum port. At idle the needle reads 18.5-18.8 inches. It flutters but only within that narrow range. If I open the throttle, on acceleration the vacuum drops to 5 inches or less depending on how quickly I pull the cable, and on deceleration it climbs to 23 inches. According to Petersen's repair manual, a piston ring defect would be indicated by a deceleration reading less than 2-5 inches above the idle reading, but my reading is right in that 2-5 range.

Another note: lately the idle has been really rough when cold, but fine when warm. Previously the idle was pretty smooth all the time. Today I repeated the vacuum check while the engine was not yet warm, and I noticed that the idle improved dramatically when I opened up the vacuum line from the intake manifold. Does this suggest that the choke is not coming off the way it should?