On the cameras that are dropping out it could be cable connections, IE CCA cable, electrical noise on the cable (too close to AC wiring carrying heavy loads or even near a strong RF source), non-standard color coding (568A or 58B are the standards), bad crimps or minor corrosion in the RJ45 itself. In this case, with that many cameras dropping at once, I'd suspect power. The easiest way to test is to use a known, good, properly terminated, cable and test one camera with that cable. If it drops again, it's more than likely the PoE switch having power problems. If it doesn't drop you have a cable problem.
Are the cameras involved outside? Were the RJ connectors properly sealed, IE dielectric grease on the RJ connector, gland properly installed and sealed, self amalgamating tape and quality electrical tape? Even the tiny amount of moisture generated by heating and cooling (day to night) can eventually result in enough corrosion to mess things up on you.