What shuts off at random times?
I can only reccomend that you narrow it down to one car, be it one of your cars or this 240z. I dont know what your financial position is (parents give you money, you have to work for it) but I just turned 16 and working on my car has been kinda hard if I needed to buy a part. Narrowing it down to one car will focus all your energy and money into making one good car. If I were you Id mabey sell 2 of your cars and put the money into the 3rd car or save it and then save up more money for the turbo 240 if it was really important to you.
Just the biggest thing I can enforce is to stay focused on one thing and keep at it.
Mabey you could get some pictures of this 240.