The issue is pressure loss. Your effectively pumping into a closed 1/2" pipe with two 4mm id outlets in it.. The pump wont manage 160gph or 4ft head with 4mm id pipe ;) Running less than 30gph flowrate thru 2ft of 1/4" pipe will exceed 4ft head, which is 1.77psi

Thanks Hex2006. I have seven 1/4" outlets just didn't draw them out. I'm also experimenting with returning some of the 1/2" pipe water back into the reservoir via a 1/2" shut off valve. So far I haven't found a setting that works.

I read somewhere else about cavitation at the pump, not sure what to make of it.

Would a more powerful pump solve the problem? Or should I just use a timer, as the pressure loss seems to reset itself after each power off/on of the pump?

Thanks for all your guidance.

cavitation is caused by entrained air, it shouldnt be an issue unless the return water is creating a lot of bubbles that are entering the pump inlet ;) You didnt provide much info about size of the system, could be you have 50ft of 1/2" pipe there lol. Are you lifting the water any distance before it reaches the 1/4" lines? how long are your 1/4" lines, super short or a few feet?

If it is pumping then stops I do not think cavitation is a problem unless your rez is too small causing then pump to be out of water as soon as the system is in full flow. Maybe faulty pump or it has sucked up some debris from bottom of rez. I suggest taking pump apart and looking at filter and for foreign objects(i take mine apart and clean every so often to inspect). If that does not help place in clean five gallon bucket or something to test. Could just be faulty.