My VX idle bounces when rolling in neutral

[djenyc]

I'm puzzled by bouncy idle on my 1994 VX - It idles fine most of the time, except when rolling in neutral (e.g. coming to a stop light) the idle bounces up and down between 800 and 1200RPM; as soon as the car stops completely, the idle stays steady at around 900RPM.

A bit of history on that VX:

When I got the car this past winter it had bouncing idle at all times after the car would warm up - I replaced O2 sensor, spark plugs, cleaned EGR, cleaned IACV and cleaned throttle body and checked resistance of TPS with no effect on the idle.

A few months ago I got a junk VX engine and swapped IACV - that fixed the idle when the car was not moving, but still had a problem when the car was rolling in neutral.

A month ago I picked up 92 VX that is Canadian model; it idles fine. I swapped IACV between 92 and 94 VX. After the swap - 92 VX continued to idle fine; 94 VX started to have bouncy idle when at stop AND when rolling in neutral.

Now I swapped IACVs back and 92 VX continues to idle fine; and 94 VX now idles fine at stop, but bounces when rolling in neutral, so that condition is repeatable ...

I made sure to drain air out of coolant lines when doing IACV swaps and checked coolant level. What can this be?

Thanks

Ross