If your water pump was replaced, then hopefully a proper coolant flush was done. If not, I would definately start with that. If they system was not flushed and previously contained large amounts of residue, which was disturbed and shaken around by adding the new coolant, this could easily cause poor radiator cooling capacity and even intermittant clogging.

Remove the thermostat, flush thouroughly with a good cleaning agent, and install a new thermostat and new coolant. At minimum that should be done to ensure your cooling system is functioning efficiently, all assuming your radiator is still OK.

Greg