Infiniti: Surging problem with 1990 Q45

I have a 1990 Q45 with 140K on the engine. The current problem that
started all of a sudden is this - the car starts properly and will
idle fine for anywhere from 1-5 minutes before the RPMs start to
fluctuate wildy. It will drop down to 300-400 RPM and come close to
stalling and then surge (probably the computer trying to compensate)
back up to about 1200RPM.

Sometimes it will do this for 20-30 minutes and other times it will
only do it for a couple of minutes before mysteriously stopping.

Has anyone experienced this at all? Any and all help would be
appreciated. Thanks in advance.

Christopher
com