R. Probst B-727 panel

The combination of the Richard Probst Boeing 727 panel and Charles Fox' set of FDE's makes, in my view, for a fabulous flying experience in all respects.I tried to take it one step further by incorporating the GNS 530 gauge from Reality-XP and bumped into the following problem: the NAV1 HSI needle exhibits an uncontrollable jitter when slaved to the GNS 530 in GPS coupled mode. It looks as if the aircraft autopilot competed with the GNS 530.Has anybody seen this or can anyone shed some light on this issue?I did check the forums including the one for Reality-XP.I realize the 727 pilots knew how to navigate without GPS at the time, but the ones still flying today probably use one, even if just clipped to the yoke...Charles