ZDriver.com - On-going problems

I have a 1975 280Z that I have had on-going problems for a couple of years. It started with the rubber? injector hoses cracking and leaking gas. They were replaced and later on in the summer it began running badly so I had the injectors cleaned and one or two replaced. Now, a year later, the car is not running good at all. It may idle and run really good then suddenly begin running very rough, blowing black smoke, maybe going 5mph with the accellerator pedal nearly to the floor. It seems as if it is terribly flooded out. After sitting a while, it may or may not start. If it does, sometimes it is up and running great again. I have run the gas nearly out and put in a fresh tank of gas with injector cleaner with no benefits. My mechanic has put a fuel pressure gauge on it while it was running both good and bad and the pressure is just what the manual specifies. There really seems to be now rhyme or reason to the problem as far as the temp or weather conditions. The fuel filter is new and clean. Can anyone help? Thank you!