  Motivated by the recent discoveries that six Ultraluminous X-ray Sources
(ULXs) are powered by highly super-Eddington X-ray pulsars, we searched for
additional pulsating ULX (PULX) candidates by identifying sources that exhibit
long-term flux variability of at least an order of magnitude (a common feature
seen in the 6 known PULXs, which may potentially be related to transitions to
the propeller regime). Expanding on previous studies, we used the available
fluxes from XMM-Newton, Swift and Chandra, along with carefully computed upper
limits in cases of a non-detection, to construct long-term lightcurves for a
sample of 296 ULXs selected from the XMM-Newton archive. Among these 296, we
find 25 sources showing flux variability larger than a factor of 10, of which
17 show some evidence for (or are at least consistent with) exhibiting bi-modal
flux distributions, as would be expected for sources undergoing propeller
transitions. These sources are excellent candidates for continued monitoring
programs to further test for this behaviour. There are 3 sources in our final
sample with fluxes similar to NGC 5907 ULX1, currently the faintest known PULX,
which would also be good targets for deeper observations with current
facilities to search for pulsations. For the rest of the PULX candidates
identified here, the next generation of X-ray telescopes (such as Athena) may
be required to determine their nature owing to their lower peak fluxes.
