One thought I had is that the pressure on the filter gauge goes from 10 PSI in bypass mode to about 15 PSI when the water runs through the solar heater. With a higher pressure, the pump faces more resistance and so wouldn't that mean that the GPM would go down slightly? Maybe in the higher GPM (bypass) case, since it demands more from the suction side, it might be sucking air in from one of the hose barb fittings or the union between the pump and the filter. I should check the fittings on the suction side of the pump and filter to make sure they are all good and snug. The weather has been really crappy up here so it may be a few days until I can get out to do this. I'll update if I find anything.

I see in another thread that others have problems with the skimmer basket moving and a rock in the basket seems to be the solution.

Thanks for your help Dave.

Peter