1994 Pontiac Bonneville no power car cuts off after 40mins

Welcome to the forum. How long since last tune-up? When the problem occurs, have a helper crank the engine, while you check for spark at the plugs. If that checks out, use a gage and check fuel pressure. You have to do testing when the problem is ongoing. Some of the national brand autostores used to loan out a fuel pressure gage. You can call and ask. Also, check for any possible trouble codes.