1991 Audi Quattro hot engine won't start

Hello,
I have a '91 Audi V8 Quattro that I just bought. It starts OK when cold, but won't start when the car is hot. If I let it cool down about 2 hours, it will start. There are no error messages, heat is not excessive (although I live in Tucson and it is summer, with 100 deg. F typical). My wife uses it to go to work, since she only has to start it twice a day, but I want to use it also for running errands, but can't. Please help. Thanks. Michael K.