Any ideas on this one Stu...

1989 ERST, Idle speed started dropping - after a run it would idle ok for about 30seconds, then the idle speed would just drop off and sometimes stall (had to keep foot on accelerator to keep revs up).

Checked CO at work, CO was at 7% - i corrected CO and it seemed to have sorted the problem.
About a week later the same happned again, rechecked the CO and it was up to 7% again (engine warm).

Removed and cleaned the control lunger in the metering unit, it did not appear to be sticking, the air sensing flap is not binding either.

I reset the CO again but after a period of time it will self increase to around 7% and the idle will drop.

Any ideas/advice??