New to Forum-Have Problem I would like Relay

Hi, I am the proud owner of a 1984 300GD (which started life as a 240GD and over the years and owners has become a Turbo 300GD) and being a diesel, it works. But I also own a 1986 280GE, this truck is super clean and has a problem which no one has been able to point me in the right direction. The problem: After you have driven 15 to 20 miles, depending how hot or cold it is outside, when you come to a stop the truck starts to shutter as if it wants to stall. Now what we have done so far includes tuning the engine, changing all fluids and filters plus general looking over. The engine runs in neutral as smooth as you would like at 850rpms (factory specs) and the transmission shifts as smooth as any I have driven but once it heats up and you are set at an idle of 850 you will stall so we have upped to idle to 1500rpms to compansate. Now I have chacked will serveral MBZ mechanics locally and they have all said "I have never seen a torque converter go bad", the torque converter being the last thing to think about. But I would think that if it were bad I would feel something funny as I drive the truck. Please give me your two cents, my head has lumps from banging it against the wall. Thanks Chris.