Just guessing as I don't have any experience myself with an RO unit (I have 2, but never hooked them up), but don't you think the more PSI your pushing through the less "filtered" the water will be? Like the water is going through the filters too fast to not do a proper job?
Again, that in simply a guess, but sounded logical in my head. You may want to see if you can back off your pressure just a touch so your not pegging the valve.