Before we can properly answer we will need some more information.
1. Does it do the same thing when you run from the internal pump?
2. Is it happening on the hot and the cold faucet or just one or the other?
3. Is it just the sink or the toilet and shower too?
4. Are you noticing any leaks?
5. Everything else in the rig is still at normal pressure?
There may be more, but that is what I thought of real quick. I would also disconnect and check all of your hose connections for dirt and grit in the screens. I would also hook up with either a new pressure regulator or without one and see if the problem still exists. Also, I would check the valves at the commode and sink if you have them and make sure they are fully open. Let us know what you find out.