Poor water pressure in cold line…

I have Pfister 3 handle shower faucet. The hot comes out like a fire hose, but the cold hardly trickles out. Did a complete rebuild on both hot and cold stems, and diverter today. Still having poor pressure on the cold side. Took the stem out, and turned the line on, and water was coming out of the pipe at a completely normal pressure. Where do I go from here?

Also, how in the hell do I keep the handles tight? They screw behind the little H/C clear plastic piece seem to loosen up if I wiggle them, or open / close them a few times.