MY CAR WILL NOT START WHEN IT'S HOT OUTSIDE

I have a 98 Buick Skylark. It was originally my Aunt's car, so I have owned it for about 3 years now. I got it when it only had 78,000 miles and now it's at about 101,000. It runs like a champ, but whenever it is about 80 degrees or higher outside it will refuse to start. When I turn the key in the ignition, it sounds like it starts, but then it immediately shuts down, and my "Theft System" light remains on. I am pretty sure it's something electrical with the theft system. It NEVER does this any other time of the year. When it's extremely cold outside (especially in the snowiest of cities) my car always starts! I do not use the key chain starter anymore, as well as my horn. There was another time when my car horn kept going off on it's own so we took the horn fuse out. I haven't really tried anything else, other than letting it sit until it's late in the evening and the weather is cooler--and it starts right up like nothing even happened and all my screaming and swearing was for nothing.

Please help. I'm begging anyone at this point.