Originally Posted by dieseldiehard

Try removing the gas cap (well, fuel cap - you know its a habit calling them gas caps!)
In case there is a vacuum being formed in the fuel tank, it would work against the lift pump (on the IP). Maybe your lift pump is getting weak?

In a 123 chassis I saw a pin hole leak in the fuel line result in strange symptoms like yours, ie. no power as the tank nears empty. What was happoening was the ppin hole allowed a stream of air to get pulled inot the supply line as the tank was low. With the tank full there was enough head on the tank to force fuel to weep out and that prevented any air from being pulled in.

My 124 300Dt (same engine as your SDL) has quite a bit of pressure in the tank as fuel gets used up. If you remove the cap air swooshes out and I haven't figured out if that is normal or not.