1992 Subaru Legacy

Engine Mechanical problem
1992 Subaru Legacy Automatic

We checked the alternator, battery, ect. And came to the conclusion that it is a sensor that needs replaced. Just can't figure out which one. The car wants to die when you turn on any accessories or stop at any place. Also going up hill it slows way down and almost doesn't make it. Any ideas?
May 4, 2010.

Is the engin light on? Most likely a faulty sensor would cause the engine light to come on and you could just pull the code(s) to identify the problem