The elevator I have has recently started shutting down intermittently. It loses all power to the CPU , all cards and hall PI. Turning the CPU power switch off and on will restore everything and it will run fine for weeks. No fuses are blown. I can not find any problems but,I am thinking about changing the switch anyway. Has anyone else experienced this problem?

power supply may be crowbarring. when you arrive, check power supply 1 for 120 vac input, if it has 120v and the 5, or 12 output supply is 0, then you are probably crowbarring the power supply. power supplies will crowbar when the current exceeds a certain amount, usually set by the current pot, they will not blow a fuse if designed in the circuit properly, and this can be confusing if you don't know about the crowbar effect. some power supplies have a voltage adjust, and a current adjust. If yours has a current adjust pot, then turn it up and see if that may take care of the problem. if not sure which way to turn it, monitor the output voltage and turn the pot until the voltage drops to zero, then turn pot the opposite way and recycle power that way you know which way to increase your current ..
also it could be that one of your boards is pulling excess current occasionally, but i would start with power supply.

Thanks for the help I will check the power supply next time it happens.

Also the 3 phase monitor/Phase Detector is a POS.. Can do that.

Ive seen faulty i/o and cpu switches as well

I bet it's the power supply, I've seen it many times.