The symptoms you describe can have their roots in multple vacuum leaks. Please check out the vacuum supply to the transmission. 800rpm at idle (warm engine) is a little too high indicate possible vacuum problem. Should be 500 to 600rpm. Does you car have an "Economy" gauge in the dash? If so, it should be pegged to the left side at idle. That gauge is a vacuum indicator and if it is not pegged at idle, you certainly have a vacuum leak.

At the age of your car, it is probably time for the rubber vacuum connectors to start deteriorating. Inspect all you can find and if you find one split or rotted, replace them all. They are very inexpensive.

A Chevy engine in your Benz is blasphemy. These engines are very durable and worth putting some effort into. I always thought the use of vacuum to control so many functions was a weak point.

You may also want to check out the function of the engine temperature sensor that feeds the fuel injectionj computer. There is nothing in the rules that says you can have only one problem at a time.

Don't give up. You have a fine car that is worth the effort.