standby device is rebboting,once active device went faulty F5 ltm(4000)

This is weird,

satndby device is rebooting every 3 seconds

No device can go on boot and come back that up. Are you referring 3 mins ?

Also search your /var/log/ltm with some keywords

boot, reset, restart, action

There will be logs indicating why your device is going on reboot.