Help please

My son has a 1997 grandprix gtp. It won't start sometimes. He replaced the fuel pump resister. The replaced the fuel pump. He started it drove it around the apartments stopped for his mail got back in wouldn't start. Then it started when he got it home. Started it a few times no problem.
$400.00 later and the same problem.
What could it be? Any ideas?

When it stalls does it have spark? this is probably the easiest thing to check first.

There are litterally hundreds of different things that can cause this. A little detective work will help narrow down the possibilities.

I have never understood why people would spend $400 dollars or more on items that might fix it when for 1/4 of that you can have it properly diagnosed. Just because a shop diagnoses it does not mean you are obligated to have them fix it. So you can still save money by doing it yourself. Even with the tows if you needed them your still ahead and you know what you need, and it is far more likely to fix the problem!!!

Yes, If it didn't have spark it would start at all right. I know nothing about this. I'm just trying to help him out.
Do you think it could be the ignition switch?
I'll tell him to get it diagnosed. Thats just another tow to a place that might not know anything. I appreciate the help.

No spark, No Start! A vehicle needs three basic things to start, fuel/air, spark, and compression. Anytime a vehicle does not start these are usually the first things checked. Mostly fuel and spark. Once we know which one is missing we can then start eliminating the possibilities based on data from the PCM, visual inspection of the components, testing of individual components and a little gut instinct as well.

Unlikely its an ignition switch. Again guessing is always going to be more in the long run!