You might try blocking off your manifold heat to see if the way you have it plumbed is causeing water flow to bypass radiator. If a 1/4 of your flow was bypassing the radiator you could have the flow problem the radiator shop is talking about. Also is your manifold heat lines picking up heat from the exhaust helping to super heat the water going back. How hot is your intake itself, maybe it doesn't need heat line in summer a lot of guys run a valve on the water heat lines to manifold that can be shut off in hot weather.
Many times our problem is so simple we look past it Good Luck.
