I am not a plumber. That said, my brother had a very similar problem in the house he moved into a couple of years ago. (Water backing up into his bathtub and then slowly draining back out) As his yard is close to the level of the lake he lives near and we had quite a bit of rain (several hurricanes) he thought it was water level. He also was having his septic pumped every three months. When he saw the septic tank wasn't full when they pumped it, he called a plumber. What the guy found was the vent pipe which was located by the commode, was plugged above the level of the drain from the sink and commode. He cleaned it by running a clean-out device (I don't know what type) from the roof down the vent and had a difficult time going through the blockage, which he likened to concrete. He said it was most likely toilet paper (the double thickness fluffy kind) which he said had at one time floated on the top of the water and was backed up into the vent due to previous back-up (full septic, etc). It then had hardened into a very dense plug. Since having the vent cleared, (almost a year ago) my brother has had no further drainage problems and hasn't needed to pump out his septic tank. Just a thought on something you could have the plumbers check out.
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