Obviously as the load gets higher, either the ignition or the fuel cannot keep up. I doubt a vac leak would cause this. A collapsing intake hose would.
I would make sure the spark is good , wires plugs etc.
Most likely I think would be lack of fuel, notably enough pressure. It could be a plugged filter or fuel line, or a weak pump, and prehaps most likely, a non working in tank pump. My 84 exibits a similiar problem and I'm pretty sure it's the in tank pump.
Do a current draw test on it.
If OK I'd get a fuel pressure gauge and hook it up while driving to make sure you have pressure under load.
If fuel delivery rate and pressure is OK, I's check for spark under load. Not sure how to do that though.........
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744 & 745 GLE's (IPD a-sbars)16v M46, 245 SE(IPD a-sbars) auto, 242Ti M46(IPD springs and bars), 745GL 8v auto