99 5.7L Surging Idle, Drive Only

I have a '99 Suburban, 5.7 L that has what I think of as a surging idle. When stopped at a stoplight, foot on the brake, shifter in drive, the idle will move up and down by about 300 to 400 RPM's. It does not do this if I shift in to Neutral. Had it in the shop and they didn't know what it was. They suggested I might need a new intake manifold. Never thought of the manifold as a wear item. Gaskets maybe (already replaced the infamous vortec gaskets a year ago at the same shop) but not the manifold itself.

I'm thinking some sort of vacuum leak, but why in drive only?

Any thoughts??