If you are seeing 4p 1.5 ips in straight and level on an intermittent basis, try measuring swashplate uniball for clamp up, may have wear on the outer races. Check Bifilar weights, pins and races for wear or grooves. If you are not having luck with rotor system, do not forget airframe and check horiz stab. for security, play, etc, have seen them loosen up over time with worn wedges. Seats and seat tracks for security. If no luck, cowls and floor panels for security, landing gear doors for looseness, and even landing gear for worn components by putting the aircraft on jacks and jiggling the gear to look for loose pivot and attachment hardware.
