As GpsFrontier points out, there are a large number of possibilities to your recurrent pH fluctuations. To me, the likely culprits are:
(1) Most-likely: WATER. Depending on the number of plants and their maturity in the system, they are probably drinking water at a much faster rate than taking up the
nutrients. If your temperatures have been relatively-warm lately, that would increase the speed of this dynamic.
(2) Moderately-likely: BUFFERS. If you are using additives to adjust the solution pH, the buffering capability of each differs widely as to how long it can hold your solution at a target pH. It is possible that some chemical(s) in your
nutrient solution or additives are unstable and lose their ability to buffer the pH after three days.
(3) Least-likely: NUTRIENTS. All nutrients are consumed at different rates, nitrogen being one of the fastest. However, this depends on which type of nitrogen you are using (i.e. urea, ammonium, nitrate) and the temperature of your solution. If you are using a mixture of these types, the pH will naturally fluctuate regardless of the addition of buffering additives and temperature as each is consumed at a different rate and urea breaks down into ammonium and nitrate.
That said, I would recommend that you start your diagnosis with a calibration of your pH testing equipment/method; proceed by trying to analyze one possibility at a time (I know this can be difficult given the interdependence of all these factors). Standardize your water level, top off your reservoir before testing your nutrient solution pH/tds/ec, and check out the composition of your fertilizer (esp. nitrogen).
Of course, I could be completely wrong - my wife would tell you it isn't unprecedented.
