Non-radial pulsations have been identified in a number of accreting white dwarfs in cataclysmic variables. These stars offer insight into the excitation of pulsation modes in atmospheres with mixed compositions of hydrogen, helium, and metals, and the response of these modes to changes in the white dwarf temperature. Among all pulsating cataclysmic variable white dwarfs, GW Librae stands out by having a well-established observational record of three independent pulsation modes that disappeared when the white dwarf temperature rose dramatically following its 2007 outburst. Our analysis of $HST$/COS ultraviolet spectroscopy taken in 2002, 2010 and 2011 showed that pulsations produce variations in the white dwarf effective temperature, as predicted by theory. Additionally, in May 2013 we obtained new $HST$ ultraviolet observations that displayed unexpected behaviour: besides variability at $\simeq$275 s, which is close to the post-outburst pulsations detected with $HST$ in 2010 and 2011, also the white dwarf exhibits high-amplitude variability on a $\simeq$4.4 h time-scale. We demonstrate that this variability is related to an increase of the photospheric temperature, argue against a short-lived accretion episode as the explanation, and discuss this event in the context of non-radial pulsations on a rapidly rotating star.