In article ,
"Set Square" writes:

It sounds as if you sometimes have a depresion (partial vacuum) in your
waste system which pulling (sucking dry) one or more of your U-bends or
traps.

When this problem occurs, does the water level drop in the toilet? What sort
of traps do you have on the bath and basin? You may be able to fit deeper
ones which are less likely to be pulled.

Bath ones are often particularly shallow and more prone
to sucking empty. Do any of them girgle away when you
finish draining the sink, bath, or loo?
Also, if this is a vistor bathroom and might go unused
for a while, traps can dry out.

Your soil stack may be inadequately ventillated. Does it go up to roof
level, where it is open to the atmosphere? If not, how is it vented?

You can get air admitance valves to prevent u-traps being
sucked empty. Either stand-alone to be inserted in the waste
pipe (and bigger versions for soil stacks), or integral with
U-traps.

--
Andrew Gabriel