Could have a blockage somewhere, also did you replace the hoses. If you have a weak hose it can and does collapse under load which will restrict flow. I'm guessing that you've replaced the cap since you did all that other work. If you have an air pocket it can cause that too. See if by turning on the heater when it gets that hot if it helps. That usually is the tell-tale sign of poor flow. You may want to see if it is the pump after a while the fins can corrode reducing flow.
