My 91 Buick Park Ave ultra loses power when I turn the headlights on

Unless you can actually feel the difference while driving - that is most likely just the extra load being placed on the alternator by the headlights - which will change the sound of the engine when they are switch d on / off - and yes it does take just a small amount of additional engine power to operate any of the accessories - however it shouldn't be noticeable while driving down the highway. It's the same with ALL vehicles equipped with this type system.