You could have a vacuum hose that has become disconnected.
You could have vacuum hose(s) that are cracked.
You could have a bad seal at the intake manifold gasket. I had an idle that went up and down, and my leak was at the intake manifold seal.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".