Change the thermostat and flush the system using rad flush, Put a hosepipe on both sides of the heater matrix and flush through. If that fails recore the original rad. Just done all the above last summer for my
408 and it doesn't move above 85 now even when stationary in a long taffic jam. And don't forget good quality anti freeze. I think that a lack of antifreeze had caused the gumming up of my rad -- previous owner. Good luck. Greg
p.s. I have spare original fans in the shroud if you need one, but I heard that the pair from a peogeot
407 diesel work great