I agree that your problems are vacuum related. If your "economy" gauge is pegged to the right much of the time the leak is massive.

The most overlooked vacuum fitting because it is hard to see is one at the back (firewall) end of the engine. It is located on the intake manifold near the center. The rubber connector for the vac line often deteriorates because it lives in a heat and oil-rich environment. If you look at the multiple vacuum lines that enter the firewall on the driver's side and trace the line that feeds them across the top of the firewall. it enters the intake manifold through that connector. You can use a piece of plastic tubing as a temporary fix until you get the proper connector. You need tiny hands and a lot of patience to reach in there. While you are under the hood, inspect all the other rubber connectors you can find also. The hard plastic lines seldom fail but the connectors do.

This fixed most of the driveability problems on my '88 560SL. Good Hunting!