Hmm, pump is clicking on and off again, now, what could be the problem now? Check valve installed, no leaks, odd.

Check the solution from UJ from post #7 or ...... as someone else will suggest buy a Hitachi

Drain the tank and report back.

The other possibility is you are bringing air from the main supply and the pump is becoming partially airlocked.

If your Mitsubishi is cycling without what you think is a leak, you should investigate all your fittings to the toilets sinks showers (if possible). I have had 8 Mitsubishi pumps & 2 Hitachi's & they are pretty much the same. If it is short cycling you most likely have a very minute leak somewhere. But the other posters are 100% correct - it is unwise to take your water directly from the main without a holding tank of some kind. We were forced to use the water in Pattaya off the main when the 15 year old(or however old it was) water tank (the blue bullshit ones) bottom broke. Soon after we had goo blobs or tar blobs coming into the water system. We moved because she refused to fix her crap. But it isn't to smart to take your water directly out of the mains. But that is up to you if your comfortable with the possible health hazards associated with doing it that way. Unless the contractor did the usual Thai way not using enough glue or starting a fire to bend the PVC pipe to fit instead of planning the run, you most likely have a minuscule leak on a hose to a faucet or toilet shower. I had my 3rd house set up & tested to 150 psi(which is a huge amount . It tested fine. a year later we put in some new faucets & one was just a tad loose(because of the great Thai threads that never quite match. Took 4 months to drip enough to catch it. Tightened it up short cycling stopped. We had a spare 305 Mitsu & it short cycled the same both now never short cycle since we located the leak. Hopefully it is not like the house we rented in 2005 . Son a bitches put the PVC in on an angle (under the floor tile) & used flame to bend the pipe. Guess where the leak was?