I have a 1997 Pontiac Bonneville that is shutting off while I am driving. It eventually starts but it happens over and over. I changed the fuel relay switch but it still happens. what next?

How long since a tune-up? Are there any applicable trouble codes? Some of the national brand auto stores will check codes free. Does it stall at idle or going down the highway?
With extended crank, have a helper crank it while you visually check for spark at the spark plugs. If spark everywhere, use a gage and check proper fuel pressure and check fuel injector pulse.

If injector pulse is missing when problem is ongoing, two things I think of, do you have an rpm signal and security system where applicable. The rpm signal comes from crank sensor. Intermittent problems can be a pain to track. Don't replace anything unless your testing points in that direction.