Vacuum reservoir. On Renix XJ's, it's in the bumper, I think it's front and centre on later models. It's either a ball shape or a cylinder with rounded ends.
It won't register as a "leak" when it fails, but there is a check valve in it that holds vacuum (when it works.) When the check fails, it won't hold vacuum and you will get the symptoms you describe with no leak to be found. Replace it.
Since the plenum dampers are held with constant vacuum, a loss of the vacuum cannister will cause the shift you describe. I have the same issue in my 88 and my 89, but I just do a quick idle to return th "heat" when my face gets too warm... I guess I should fix those one of these days...
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