1990 Buick Riviera car dies

When it dies, can you restart it? If not, have you checked for spark and fuel to the engine at that point?

Yes it will start back up but I have to wait for a while, it does have spark, and it is getting fuel

You mean it when it dies and won't restart, it is still getting spark and fuel? Are you certain? Is the spark a blue snapping spark and fuel pressure within the manufacturer's specs?

Your right after it dies there is no spark until it cools down but it is getting fuel.

Okay... First, I am going to tell you what I think the problem is based on past experience. What I believe is happening is that the crankshaft sensor is most likely cracked. What I have seen happen is when the engine is cold, the sensor works. As the engine heats up, the sensor expands and spark is lost.

The only way we can be 100% sure that this is the problem is to have a scanner that can read real time data. When there is no spark and it won't start, we need to know if there is a signal from the crank sensor. No signal = bad sensor.

I replaced the crankshaft sensr and now it backfires and cuts out and has loss of power so what would cause that.

Many things could. Have you checked to make sure timing is correct? Have you checked fuel pump pressure to confirm it is within the manfacturer's specs even though it is new?