Black wire doesn't mean its an earth mate , Id take the advice of the long standing members of the forum ,take it to Stechnic They know their stuff back to front Leave it with them for a few days let them diagnose and fix the problem and pay them .
Most of us on here that do our own work know these cars how to test have the right tools, diagrams and books .Without seeing the car and just giving advice its like sliding down a 50ft razorblade using your balls as the brakes (painfull).
It seems a few different people have worked on the car . Best to take it to one shop ,get them to fix the problem , go over the car make sure its tip top then tune your car so it lasts and puts a smile on your face .
It will cost money , but its money well spent and at least by getting the car serviced and tuned at the one place they get to know you and your car which in the scheme of things makes life easy not hard . Then make sure you use the same place for re-tuning ,mods and service YOU CANT GO WRONG!
But you need the blank canvas to start with until someone goes over the car and fixes the problems one thing will only lead to another and the Fuji Heavy Industries Leprechaun will rape your wallet .

good luck