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Idle keeps on reving when stoped

I have a 90 nissan p/u and after the engine has been running a while,and I stop at a light,and the gear is in neutral,the rpm will go from 1000 to 2000 rpm every 2 seconds.When im driveing it’s ok,or when im stopped but have the ac on it’s ok.I have had it in at an auto electric shop and they can’t figure it out.They replaced sensors in the radiator that might be telling the computer that the engine is cold,and various other things.They tell me that it must be the computer,but they have no way of determing if the computer is bad or not.They said that if I bought a new computer for $600 and then found out it wasn’t the computer,that I wouldn’t be able to return the computer,and I would still have the problem.So what can I do?

Can you pull a computer out of a salvaged truck and try it? Perhaps a different shop could install a new computer and if it doesn’t solve the problem, put yours back in for just the labor cost.

They’ve ruled out vacuum leaks and idle air control problems?

Why the duplicate post? Didn’t like my answer? Why didn’t you say so?