what is wrong with me 2.0L vectra 2000?

hi, i have a problem with my water system it seems like it is air locked. after i have been out driving it the water bottle is bubbling and trying to over flow, but when i rev the engine the water goes down to normal level. but when i turn it off it will try and over flow and bubble out.
things i have done to try and fix it....

replaced the raditor
flushed and cleaned all the pipes
flushed the heater matrix
replaced the thermerstat

many thanx