[V8] No start sometimes

[george.fallot at verizon.net]

  Once again I need help.I'm hoping someone on the list can help,if not I'm thinking of the cash for clunkers program. I hate to do it but I've lost my confidence in my V8.
  Last Nov. I had a small fire in one of my '90 V8's. It had 165k and I couldn't get it running again, so I took what I could off it (which was almost everything) and scrapped the rest.
    In June the tranny went on my other '90 V8, 97k. My tranny guy was willing to swap it for me for $500 as long as I wasn't in a hurry. It took 6 weeks, but I finally got it back a couple of weeks ago.
   It ran great for 1 week, about 300 miles,and I remembered why I like these cars so much.
   But last week it would not start. It was in my driveway and not hot. I determined that it wasn't getting gas. I gave it a bit of starting fluid and it started and ran well. But if I turned it off it wouldn't start without the starting fluid.
   I changed out the two sensors (speed and crank) with my used ones and it started right up. I used it for 50 miles and started and stopped it about 8 times.I let it sit for 5 days and today it started right up. I went about 4 miles, turned it off, shopped for 10 minutes and it would not start again.
    I got one of my sons to bring me a can of starting fluid and it started right up. I drove it home and it will not start without the fluid again.I'm at a loss,I've owned and worked on these cars for over 10 years now, but now I'm not sure it's worth it, as I don't know if I can depend on it anymore.
    Hopefully I've given you guys enough info that it rings a bell to someone, sorry for the long post but I am really upset with the car today and my lack of knowledge about it.
  George