I have had issue of the GS4 asking for a reboot after "SIM Card Not Detected" error.
Nothing to do with ROM, cache clearing, SIM itself, anything anyone has brought up. I've tried it all.
After reboot, it will work, until the next time, which can be hours, days, or withthin minutes, has nothing to do with heat. Happens even just sitting in winter.
It used to not do this.
So I opened the phone, cut a small 1/8" thick piece of soft ear plug foam (not the hard yellow/orange) bought from Auto parts store for $1. The form will compress considerably and fit in just fine.
I put it on the connector plug for the SD/SIM board. If you look, Samsung out in a thin pad to apply pressure on that connector. But it was really thin.
The connector is next to LED Flash and right above the SD card slot.
Now after more than 24 hours I have no more "SIM not detected" errors. Before it will crop up multiple time everyday without fail.
I hope this works for you all. Remember. It must be soft ear plug foam. It compresses and expands as necessary and applies pressure equally. Guitar picks or paper will not work as well as foam.
My guess is that after normal use, heat, dropping phone, etc. The tiny connectors are not very secure anymore. So even if the boards were replaced, if the connectors are not pressure secured, they won't make good connection and loosen itself.