Momentary rpm surge from idle

[Brian Sims]

I have been unable to find a cause for an intermittent rpm surge in my 1990 type 44 with an NF motor.I hope someone else has experienced this and found remedy.It only occurs at idle,in or out of gear  and is only momentary.There are no codes and all the usual diagnostics passed.I have noticed that it does not occur during warm-up and the frequency valve to the charcoal canister stops for 15 -20 seconds after each incident.It is initiated by a momentary signal loss to the Idle valve but I don't know what sets it up.Please advise.