500' should be enough.
The other possibility is that the repeater itself has some desense caused by it's own transmitter. If a weak signal opens the receiver, causes the controller to key the transmitter, the repeater's transmitter then desenses it's own receiver, the receiver squelches, the transmitter drops, and the receiver hears the input signal again.
This can cause a repeater to just sit there and go kerchunk kerchunk kerchunk and on and on as long as that weak signal is present on it's input.
Either way, I still suspect desense, it's just a question of which end of the path it's happening on.