I has the same problem or similar.
The idle speed would be around 2200 rpm. If it hit 2250 it would jump down to 1500 and oscillate between the two.
The problem was that my radiator had leaked and it was low in coolant. Not low enough to see the engine temp rise but low enough to effect the idle speed.
I guess there is some sensor or vacuum which links the idle speed and coolant temp together
Now I have a new radiator and it is working perfectly.
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