Thanks all for the replies and help!
The screen at the pump intake is clear and the line from the tank is already a reinforced--it is this product and doesn't seem to be collapsing under the suction--at least from a visible/tactile inspection. It is made by the manufacturer of the pump, too, so hopefully they knew what they were doing...
I still think the problem is between the tank and the pump, though, and I'll be taking the lines off and pumping out of a bucket once I get a chance to see if that improves things vs pumping from the bottom of the tank,
The pump is supposed to be able to dry prime up 6' and the way I have it mounted, it is at the same level as the tank. The pressure is low, even if the water level is above the level of the tank.
At this point, the most probable cause to me, would be some kind of air leak on the pump intake or a hose that is collapsing. Once the tank is empty, I'll toy with different hoses and a bucket of water to see if that helps. Like I said, I already replaced the pump and I have checked in the tank for obstructions. It's always been this way and I am pretty sure that the pump is getting to 55 psi when the faucets are off, but once a faucet is open, it can't keep up. If I turn on the water in the bathroom, the kitchen sick doesn't even work.
It's just hard for me to imagine why it would be having such a hard time sucking water from the tank unless there was a pinhole vacuum leak in the intake line or an obstruction like a collapsed hose. All my fittings are 1/2" minimum between the tank port and pump.
Maybe I'll try and blow out the line between the tank and pump as well....
Thanks again for the ideas, guys!