HELP : E420 Running problem
 

I've this 94 E420, As a maintenance, I replaced fuel filter, spark plugs, wiring harness, air filter, oil 2 filter but my problem persists. It runs great for 3-4 miles then suddenly starts shaking when stopped and does not pickup at once. It does this for 3-4 miles and then runs normally. I don't know what the problem could be. I don't know where to look for. Any help is appreciated

Thanks in advance

many possibles
 

cold idle sensor,mass air sensor,tired o2 sensors,partially blocked cats,egr valve.
You need to get it on a scope so it can be diagnosed what happens when and what sensor or unit is dropping the ball.