  (BEDT-TTF)$_2$MHg(SCN)$_4$[M:K,Rb,Tl] shows typical two-dimensional angular
dependent magnetoresistance oscillation (ADMRO) at high temperature (T$>$8K),
but at lower temperature it shows anomalously large magnetoresistance, and the
ADMRO pattern changes. These low temperature behaviors are explained as effects
of a periodic potential. The present explanation is different from that by
Kartsovnik et al. [J. Phys. I France {\bf 3} (1993) 1187] in that
reconstruction of the cylindrical Fermi surface into an open Fermi surface is
not assumed. It is also predicted that if the periodic potential exists at
quantizing magnetic field, resistivity peak of new origin should be observed.
