The flow of water frequently will fluctuate. It will drop some for a 5 or 10 seconds and then resume the original rate. This occurs over a much shorter interval of time than the typical cycle when the pump comes on at 30 PSI to boost the pressure in the tank back to 50 PSI.

Our system is 12 years old. We have a water filter after the pressure tank that needs changing every 3 or 4 weeks to maintain adequate flow.

What are the various most likely explanations for the fluctuations?

(I don't know if mud or sediment has collected inside the pressure tank, but if it has, could that be a source of the problem? I have always assumed that the regular flow of water into and out of the bladder would keep it fairly clean.)
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speedball1

When that happens in my area it is usually caused by a low level in the water table. Is there a bypass on your filter that could be kicking in because the filter's clogged?
Any sediment in your system should pass through the bladder tank and be caught by your filter. Any number of things could cause this. Faulty impeller or pump, ect. Let me know what you find out. Regards, Tom

Ed Graham

The problem turned out to be a ruptured diaphragm in the pressure tank. Looking back on what I have learned over the past couple of weeks, I should have recognised that as the obvious answer.

I had the tank replaced today and the system is working well tonight.

Ed Graham