Does an erratic idle do any harm?

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I just did the obd1 conversion and car runs fine, except when im slowing down and let my foot off the brake, the idle goes between 1000-2000 every few seconds. ANy idea what could cause this and is this really a bad thing?
That happens when the ECU is struggling because there is too much air entering your engine. Probably a vacuum leak somewhere. Does it do it all the time? When the engine is hot? Cold?