1999 Honda Civic My car turns off

My car turns off randomly but it mostly turns off when the weather is really hot. It wasn't so bad, but it keeps on getting worse and worse. It use to turn off and turn right back on but now I have to wait a while because if I try to turn it on right away, I just watch it turn on and off in a second. I always have to keep it in the shade if I want to be sure that it will turn on. Recently, it turned off like twice at night --due to its progressive nature. I've gone to the mechanics, but they were stumped, and when they tried doing something about it and charging me $170 for it, that's when it started getting worse.