pioneer deh-p47dh

I have an unusual problem. I installed pioneer deh-p47dh in a 97 camaro convertible. I start car all is good everything works. I stop and shut off car then restart and stero doesn't work, power windows don't work and conv. top loses power as well. I drive for a while and stereo turns itself back on and windows and top work again. as I'm driving some times the stereo and power to windows and top will go on and off several times. Other times everthing works fine. I bought the stereo from crutchfield and it came with wiring harness to match camaro. I tried another unit, brand new and same thing happens. I replaced the complete wiring harness and the plug that goes into the stereo still happens. I remove the stereo and reinstall the factory delco stereo and problem goes away completely. The funny thing is that the pioneer stereo when I originally installed it worked flawlessly for almost a year.
As I said I even installed a brand new stereo of same model and still had problems so by process of elimination I have ruled out the wiring harness and stereo as being the problem, however If the car itself has a problem then why does the delco stereo being installed eliminate the problem? I need some help. thanks