Sounds like a partical blockage starting Or poor venting.
Ever take a 2 liter bottle pour it upside down? PLOP PLOP PLOP now put a small hole in the top and then try pouring it
Wow equalized pressure and no vacuum. That is what a proper vent does.
Check your vent terminal (roof vent) and then possibly have your waste/soil lines water jetted to remove old debris build up restricting the full flow on the internal piping
[Edited by SylvanLMP on 14 March 2001]