2 stroke help, please! RPM's lower, then bikes quit?

This has happened with 2 of my vehicles. A 1997 Polaris 400 Sport quad, and a 2004 Honda CR125 bike. They will start fine and I can take them out for a quick rip, but after 2-3 minutes, the bike/quad will slow down, RPMs will lower, then the bike will die and not start again, and even sometimes backfire. It couldn't be a faulty top end, because the 125 is coming off a fresh rebuild. It couldn't be the carb, because on the Polaris, it has been torn apart and cleaned more times than I can count. The plugs are brand new.

Leaking reed valves.
Remove the reed block and hold it up to the light.
You should not be able to see any gaps between the reeds and reed block.