Throttle actuators are a part that wear out over time.
Stevebfl said that the potentiometers wear out.
euro246 said that the harnesses on the actuators go out, too.
The dealer or a good independent should be able to test the part.
I wouldn't experiment on your own, though, with a new actuator. If you install it and it doesn't fix your problem, you can't return it because it has been installed. If the dealer or an independent installs it and it doesn't work, they'll just keep the part as inventory and use it for the next time.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".