  We study the problem of Arnold's diffusion in an example of isochronous
system by using a geometrical method known as Windows Method. Despite the
simple features of this example, we show that the absence of an anisochrony
term leads to several substantial difficulties in the application of the
method, requiring some additional devices as non-equally spaced transition
chains and variable windows. In this way we are able to obtain a set of fast
orbits whose drifting time matches, up to a constant, the time obtained via
variational methods.
