i just replaced the ecu in my pickup. it would start and run fine and then just die. few minutes later it would be fine again. get a new one and keep a spare in the glovebox. they do "just go out", its pretty common. make sure that it has a good ground. scrape the paint off where it bolts to the body just to make sure.
one other thing you might check is the gap at the pickup in the distributor. there should be a gap just like points. im not sure what the gap is but someone else will.