Hi all, sorry for the delayed post. Water in the system is as clean as it gets, also running with Sentinel inhibitor for as long as we've been in the house. The plumber has since clarified that he thinks the pump is failing (rather than the original suggestion to upgrade to a more powerful pump). I can see how this would cause the problem but the pump isn't noisy in any way, so not sure if a pump can fail silently. I guess it can?