My '88 cranks, runs for 5 seconds then dies

If I continue cranking it might fire but won't run. I have been told that the problem could be the key cylinder is worn and the key is making a poor contact so I have bought a new lock cylinder and new key with the same ohms reading as the original to replace.

If it is not cranking then you may have a problem with the VATs that could be related to the key and cylinder.  It would act like a dead battery.......... But this is not your case. Replacing the ignition is not going to do anything for you.

There's a very good article in the "Technically Speaking" section of Apr. issue of Vette that you should read. They go through all the reasons fuel & electronic, most of which you can check yourself. Actually scary how many things need to go right compared to a carb and distributor! Good luck, and for what it's worth, my money is on fuel pump ...

Thanks for the advice, I'm just reading that article now. I replaced my fuel pump with a new Bosch pump approx 5 years ago, at the same time replaced the fuel pressure regulator and the oil pressure switch behind the distributor. The car doesn't get much use maybe that's part of the problem.?

Thanks for your advice. As you can see someone else also offered the same diagnoses...I appreciate your comments.

I would start with putting a pressure gauge on it to see what the fuel pressure is. The pump has to be running, or it wouldn't start to begin with. That being said, it doesn't mean the pump is putting out the correct pressure, or it might stop running after a few seconds. A gauge would narrow down the list of possibilities. If it starts and runs for that 5 seconds, it could be the oil pressure sensor. The fuel pump runs off of a relay to start, but it requires oil pressure to keep another relay energized after the initial start. See what the fuel pressure is doing first, then go from there. 

not to be a smart assssssssss. you just could be out of gas.  ive ran into that before. the gage was way off.

I'm tyring to get a hold of a fuel pressure guage. I have been told that their are two oil pressure switches/sending units, I know about the one behind the distributor [which has been replaced earlier] where is the other one.??

You have one sender unit by the distributor, and the other is just above the oil filter. I had a pic here somewhere, but now I cannot find it. 

As a test, you can by-pass the one next to the distributor by using a jumper between the terminals. This will keep the fuel pump running IF the oil pressure sender is the problem. The sender by the oil filter is normally only used for the oil pressure gauge itself, and should have no effect on the fuel pump running.
The sender you want to by-pass will have an Orange wire, and a Red wire. Jumper these two terminals together, and it will eliminate the sender unit as the problem.
There are other things that could cause the same symptoms, but this is fairly easy to do quickly.