Did you reflow pin 13 of the connector? That is the VSS input to the computer. It should not impact your idle but it is a bad solder joint. The fuel cut solenoid is on pin 12 above it that looks pretty solid.
The emission computer will allow the FCS to close under conditions of hi rpm and hi manifold vacuum, like you suddenly let off the gas. Per the manual, this system cuts off part of the fuel in the slow circuit of the carburetor to prevent overheating and afterburning in the exhaust system...
As @firestopper pointed out, the emission computer controls the FCS. It is possible there is a broken solder connection on the connector that is giving you an intermittent or high resistance connection.
The 81+ emission computer is ultra reliable. I have never found one failed completely. That one shown above with broken solder joints is not an 81+. I have seen those with broken solder connections on the connector on the 79/80 boxes. It is easy enough to look at but I don't think that is...