I used the same pump in my experimental set-up at work. I've noticed the flow rate can vary. For me what happens is that small air bubbles get into and build up int the pump which reduces output. Then after a while I get a big bunch of bubbles that comes out of the pump and the Flow rate bounces back up again. This has been throwing my automatic control because the control system tunes itself to deal with the gradual decrease in flow and is then thrown out of whack when there is an instantaneous change in flow rate as the bubble leaves.
Do you hear your pump getting noisy at all?
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