I haven't seem mention of the pressure tank being the problem. They can get waterlogged and this could cause problems. Shut off the power to the pump and open whatever drain valves you have around the pressure tank until it is completely empty. Close the valves and start up the pump again and see if things improve. It also helps if you can pressurize the tank or at least check the pressure as mentioned further up this thread. For this you'll need an air pump.
I think this may be the correct answer.
Google "waterlogged tank" & you lots of good info.