I guess at least we ascertained that this is where the bandwidth was being set to 50kHz, but the centring I have no idea. Did you try disconnecting the external radio and just hooking up the RSP on it's own to check the behaviour then? Because my guess is that the re-centring is only going to happen with the external radio. It's certainly not a behaviour I've ever seen before without an external radio (not on any of my SDRs). Maybe it's even Simon's intended behaviour when using an external radio? Only Simon will be able to comment on this one.
73
Max