Recently bought a 1986 300zx non turbo a couple days ago. all original 88k miles, the guy said the fuel injectors was the problem but getting it home and looking around it sounded like a misfire so i changed all the plugs and swapped them out with the set of wires he gave me with the car and the plugs looked terrible so i thought maybe that would smoothen it out. i’ve also ran higher grade gas through and cycled it cause it did smell like old gas the guy also put a new fuel filter on the car cause he said the stock one made it sound like it was starving itself, im not to sure if this would also make the car do this or not since the one he put on looks nothing like the factory one. The car still does the same thing it has a jumpy rpm in park but will sometimes average 1100 and putting it in gear it drops to 300-600 and starts shaking really bad before it tries to shutting off and eventually will. i’m new to these types of car i usually work around american so thought i would shoot my problems here.