The radio-quiet thermally emitting X-ray pulsar RX J0720.4-3125 belongs to a group of seven isolated neutron stars (often referred to as the ``Magnificent Seven"). They share similar properties such as pure thermal radiation (black body emission with broad absorption features in some cases), long pulse periods 3-12 s) and high magnetic field strengths (B~1e+13 Gauss) on their surfaces. These neutron stars are nearby (<500 pc) and up to a few million years old (i.e. relatively young and still hot), thus, most of them have been identified with dim (25-28 mag) blue optical counterparts. RX J0720.4-3125 is unique among the ``Magnificent Seven" as it shows long term variations in its spectral properties (temperature, size of the emitting area and equivalent width of the broad absorption feature) and irregularities in the pulse period on time scales of years. The main two theories put forward to explain the behaviour of RX J0720.4-3125 are either free precession or a glitch - a sudden event like a star quake or an impact of a massive object, that released much energy in a short time. In this work, most recent data from new observations with XMM-Newton and Chandra as well as already existing archival data are analysed (altogether now covering a time span of almost twenty years). While the spectral changes clearly do not follow a sinusoidal variation if the new observations are included, the phase residuals seem to follow a periodic behaviour. An updated phase coherent timing solution was performed and it was shown that for such investigations a restriction to the hard energy band at 400-1000 eV is most suitable to achieve best agreement between the phase residuals derived from different X-ray instruments. Based on the new data, different theories to explain the behaviour of RX J0720.4-3125 are modelled, reviewed and discussed, as well as physical consequences of the conclusions are presented. The model of free precession explains well the timing [abridge]