Somehow I missed those last two comments, but thanks for chiming in.
After telling me that the transformer is $150 to replace, the shop suggested that I contact one of their lamp repair places. Over the phone, the guy said that the transformer is not likely the cause because it is pretty much a working or not working thing, not what I have been experiencing with the light going out after awhile of being on. He guessed that it might be the socket because the contacts get so hot in old halogen lamps that the solder can actually run and after years it can be minimal enough contact that after it heats up again the contact is lost and the light goes out. I will take it in to have him take a look. Can't hurt, and if it is the socket it will be an easy fix.