Context. Short time-scale flux density variations of flat spectrum rad io sources are often explained by the scattering of radio wav es in the turbulent ionized Interstellar Matter of the Milky Way. One of the most convincing observational arguments in favor of this is the annual modulation of the variability time-scale caused by t he Earth orbital motion around the Sun. J1128 +5925 is a recently discovered IDV source with a possible annual modulation in its vari abil ty time-scale. New observations suggest a change in it s variability characteristics. Aims. We search for changes in source-structure or in the interste llar medium to explain the variations in the IDV pattern of J1128+5925. Using Very Long Baseline Interferometry (VLBI), we st udy a possible relation between source orientation on the sky and anisotropy angle seen in the annual modulation. Addi tionally, obtaining a source size estimate from VLBI data en ble us to calculate the distance to the screen causing the variations in J1128+5925. Methods. We observed the source in six consecutive epochs with the Ver y Long Baseline Array (VLBA) at three frequencies, 5 GHz, 8 GHz and 15 GHz in total intensity and polarization. This dat a re combined with our densely time sampled flux density moni tori g performed with the radio telescopes at E ffelsberg (Germany) and at Urumqi (China). Results. The VLBA observations revealed an east-west oriented corejet structure with no significant motion in the jet. The expan sio of the VLBI core leads to an estimate of mild relativistic spe ed (2.5 c± 1.4 c). The position angle of the VLBI jet agrees with the angle of anisotropy derived from the annual modulation mode l. No significant long-term structural changes were observe d with VLBI on mas-scales, however, the VLBI core size expansion o ffers a possible explanation to the observed decrease of the st rength of IDV. VLBI polarimetry revealed significant changes in the electr ic vector position angle (EVPA) and Rotation Measure (RM) of the core and jet. Part of the observed RM variability could be attribu ted to a scattering screen (37 pc distance), which covers the ource (core and jet) and which may be responsible for the IDV. Superposit ion of polarized sub-components below the angular resoluti on limit may affect the observed RM as well.