I'm thinking the same sort of thing lockrockie is suggesting. Barring the possibility that someone might be topping up that you don't know about, it sounds more like to me that the speed of water in the overflow pipe is not steady state. If it speeds up the main tank drains faster until it reaches steady-state again (but by then it's lowered in the main tank, and we could be talking a minuscule amount for the level to lower for it to be substantial in the sump), and more water ends up in the sump. If it slows down the main tank drains slower until again steady-state is found and now the sump is low.
Things that can cause speed to change in the overflow plumbing are things like excessive bends that are prone to clogging (ie., snail in the pipe or whatnot), or even if the outflow pipe is submerged, it could be building up backpressure. Is there any "burping" happening where water enters the sump?
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-- Tony
My next hobby will be flooding my basement while repeatedly banging my head against a brick wall and tearing up $100 bills. Whee!
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