A cold thermostat will not force your car run cool. It just opens enough that the pump and radiator operate at their natural temp for the RPMS the engine runs. The stat actually closes to keep temps UP to 195, which is what the EFI requires to go into closed loop.
The rad hose sucking shut isn't because a new pump does a bunch better - it's the old radiator finally showing it's clogged badly and can't keep up. With overheating over 30mph, it's really clogged - or - the water pump is the wrong rotation. All the 4.0 should be serpentine belt, but ya never know who slipped in the wrong impeller or a 258 pump - V-belt rotates backward. They use the same gasket, too.
If your radiator is more than ten years old, then get it replaced with one having an additional core for more capacity. Rodding it out is a short term stopgap, aluminum rads with plastic tanks can't be. Check the pump for correct rotation - means pulling it, I know.
