You should check your typing or tell me where you found a 96 SDL

Easy thing to check is a clear line from the intake manifold through the switchover valve to the ALDA, that squarish thing sitting at an angle atop the injection pump.

Does your car have a trap oxidizer over the turbo? It's a 10" diameter metal can. If it's more like a 4" diameter metal can, you don't have a trap oxidizer.

Check the exhaust system. If there is something that looks like a catalytic converter near the transmission, it's likely an oxidation catalyst and those are known to plug up. It gets so bad that the car hardly moves and then can't start. Once it gets to the point that the car is hardly driveable, remove the plug on the #1 exhaust runner to see if there's any improvement. If the car eventually gets to freeway speeds, it's too early to use that means to test for a plugged catalyst.

Sixto
87 300D