Chevrolet Repair: 1997 tahoe wont stay running, oil sending unit, fuel rail

Question
hi,    I just installed a new 5.7 in my 1997 tahoe. I had help from the mechanic at my work. after making sure all wires were correct and adding all fluids, I started it up and it started immediatly and sounded good. it was runing for about 3 minutes and then it suddenly shut off . I tried to start it and it popped and backfired but didnt start. I was scratching my head wondering why it wont start again. about 30 minutes later I tried to start it again and it started immediatly and ran again for about 2 - 3 minutes and the same thing as before. I replaced the fuel pump (because I still had a warranty from the last one I bought) and still the same thing. I sprayed starting fluid and it just dieseled but didnt start. so, heres what I know    it starts when cold every time  and a couple of times I had one foot on the brake and the other on the gas not letting the RPM go below 2000 and i could drive it. when I did drive it (about 45 mph) it was perfect  no missing lots of power and no backfire  but when I slow down thats when the problems start... I hope this enough info for an answer.     thank you very much  

Answer
Your problem leaves me with a lot of unanswered questions. I would first check the fuel filter. I would then check the line pressure in the fuel rail and then see if the fuel regulator is functioning. If you have access to a scanner watch for the command to the fuel pump in the data stream to see if it is intermittent. I believe that on that vehicle the computer must see oil pressure to continue to let the fuel pump run. You might check the connection to the oil sending unit. There should also be two fuses to check...one drives the fuel pump when you ''crank'' the engine ,the other allows the fuel pump to continue to run after the vehicle is started. The vehicle may be initially running off the ''crank'' pressure in the fuel rail for the first 3 minutes...I hope this helps...Best