at irregular intervals
at random
brokenly
by catches
by fits
by jerks
by snatches
capriciously
desultorily
disconnectedly
discontinuously
eccentrically
erratically
fitfully
flutteringly
haltingly
in snatches
in spots
inconstantly
intermittently
irregularly
lurchingly
nonuniformly
off and on
patchily
quakingly
quaveringly
quiveringly
roughly
saltatorily
shakily
shudderingly
spasmodically
sporadically
spottily
tremblingly
tremulously
uncertainly
unevenly
unmethodically
unpredictably
unrhythmically
unsteadily
unsystematically
variably
waveringly
whimsically
wobblingly