Jim, are you staying at a very cold area? Did this problem start together with the colder months? Low temperatures could increase your fuel and/or oil viscosity index, resulting in poor fuel pumpability.
You could have a mechanic check the fuel feed pressure after the filter or at the injection pump fuel inlet. You should see approximately 14 to 25 psi both on a cold and hot engine.
If you can't get good fuel pressure, try using some fuel additives to correct fuel viscosity. Also, check your fuel banjo bolts for blockage. Sometimes, MB uses an air bleed banjo bolt with a small orifice on it. If this fitting is used on any of the fuel supply lines other than the fuel return port, your pump would be receiving insufficient fuel.
I hope this helps.