Check cam and IP timing.

Also do a balance test - with the engine loping at idle, crack each injector line one by one to see if any doesn't make the engine idle worse. If you find such a cylinder, swap injectors with another cylinder to see if the problem moves with the injector or stays in the cylinder. If the problem is with #1, it could be a blown head gasket. The good news is a new gasket will put things in order.

Sixto
87 300D