Problem with idle
 

Hello:

I have a 1996 C280. When I start the engine, the idle fluctuates and if I dont keep it accelarated, the engine dies.
Once you get it started , it goes, but at stop lights the engine dies again.
On the highway it works allright but not in the city at stop lights or when I have to slow down to enter the highway.
The first time this happened, it was a very hot day (100 degrees).I went to a Mercedes dealer who checked the codes but found nothing wrong except he said I have to replace the throttle actuator.
He sprayed on the top of the engine to check for vacuum leaks and it seemed like it helped because gradually the car started working better.It worked perfectly for about a month and now it is doing the same thing.
I thought the problem had gone away and now it is back .
Has enyone experienced this?
I need help.
Thank you,