I had similar symptoms recently, which progressively got worse. I had two problems with my SU's, Throttle shafts and bores were badly worn.

One throttle plate would not always close completely due to the slop (it was contacting the edge of the bore). When the Idle would "race" i could manual turn the throttle shaft back to the closed postion. a very small movement, but the return spring couldn't quite close it completely. The idle would then settle down.

Also, 1 of the jets was not centered to the needle and the piston would not always drop completely. I'm not sure what symptons that caused, perhaps worn needle/jet, fouled plugs, poor performance, and bad gas mileage.

I replaced the shafts (still need to rebush), centered the jets, cleaned and adjusted all linkages, changed the spark plugs and the problems have been gone for about 3,000 miles.

good luck,

curtis m. white
http://thirdchoice.com