Dual Quad Problem

[ERA Chas]

I have twin vac. sec. 654's on a Tunnel Wedge. They work and meter perfectly.
This is a combination that has run 15 years with minor maintenence and no changes. There are no bogs, hesitations, leaks or any other imperfections.
EXCEPT-over the last two seasons they will not return to idle while running on the road and coming to a stop. The throttle will come down to maybe 1800-1500 rpm. If I just poke the pedal it will come off and drop to my normal 1000 rpm idle-and idle perfectly.
I will eliminate all the easy answers-no vac leaks, no binds in the linkage (heim joints), all springs and geometry perfect -nothing I've checked or tried has altered the problem. I've been very methodical but I'm no genious.
I'm thinking of swapping carbs front to rear to see effect. The best suspicion I have is the throttle cross link is naturally sloppy on these (no bushings or bearings on these) and may be the source of the hang-up. If the replaced carb does it too, then that's all I can pinpoint. If I'm right, does anyone know where I can get cross-linkage which is bushed? Remember, the shaft which goes through the plate is brazed to the levers on both sides.
Any experiences similar to this and the fix if found would be greatly appreciated.