We have acquired new time series photometry of the two pulsating DB white dwarf stars KUV 05134+2605 and PG 1654+160 with the Whole Earth Telescope .
Additional single-site photometry is also presented .
We use all these data plus all available archival measurements to study the temporal behaviour of the pulsational amplitudes and frequencies of these stars for the first time .

We demonstrate that both KUV 05134+2605 and PG 1654+160 pulsate in many modes whose amplitudes are variable in time ; some frequency variability of PG 1654+160 is also indicated .
Beating of multiple pulsation modes can not explain our observations ; the amplitude variability must therefore be intrinsic .
We can not find stable modes to be used for determinations of the evolutionary period changes of the stars .
Some of the modes of PG 1654+160 appear at the same periods whenever detected .
The mean spacing of these periods ( \approx 40 s ) suggests that they are probably caused by non-radial gravity-mode pulsations of spherical degree \ell = 1 .
If so , PG 1654+160 has a mass around 0.6 M _ { \sun } .

The time scales of the amplitude variability of both stars ( down to two weeks ) are consistent with theoretical predictions of resonant mode coupling , a conclusion which might however be affected by the temporal distribution of our data .