Air carrier First Officer reported the aircraft entered an uncommanded pitch downwards while on autopilot with the glideslope and localizer captured. The autopilot was turned off; the aircraft corrected; and the approach was completed by hand.

Narrative

I am unsure what category this falls under. On final for the ILS for Runway 10R in PDX; the aircraft aggressively entered an uncommanded pitched down on autopilot with glideslope and localizer captured. We were instructed to maintain 3000 ft. until established. We intercepted the localizer and glideslope simultaneously outside of HAIRN. I immediately turned the autopilot off; corrected; and continued the approach by hand and landed without incident. There was no control malfunction and the plane flew normally after the autopilot was turned off. No aircraft; FOM; or operational limitations were exceeded. The pitch down had not developed enough before I intervened to constitute a missed approach situation. At the time of the erroneous pitch down; we were on both glidepath and localizer. Upon breaking out of the cloud coverage around 2700 ft. AGL; I noticed Aircraft Y holding short of Runway 10R at the hold-short line. I am unsure if this was a cause of the erroneous pitch down due to interference or not.