What hardware board are you using, or what is the schematic of the interface adapters you are using to convert radio signals to those compatible with the RPI?
The port 1 is a repeater, so yes it is expected that the COS would trigger the PTT to immediately go active and would stay active until the COS is released plus some small delay.
Additionally the PTT will go active when the system self identifies, and when there is an incoming echolink event.
There is nothing unstable in the default settings, this revision has been available for several years now with no reports I am aware of related to instability. Perhaps the TX radio is getting into the RX radio? I have seen this several times and will appear to be an oscillation behavior as the TX causes the RX to open which causes the TX to enable, cycle repeats indefinetely