I have a 94 Lesabre and it's been running rough on the highway, have been told by my mechanic it needs an egr valve. When I have heater, lights, signal on and brake to turn engine almost dies, is this all to do with egr valve?

Well, if the engine is already running rough, then putting a load on the alternator will bog the engine down even more and if the engine isn't running at a high engugh rpm's then it would stall. But as for the egr valve is the check engine light on? What kind of motor do you have in that buick?