David,
  Generally, I have found that when the temp starts rising in traffic and
is fine on the freeway, it means the water pump is starting to go.  Usually
if the passages in the radiator are getting plugged is a much slower process
and you would have noticed the car  running hotter on the gauge in ever
increasing amounts.

Cheers
Webserve

Check your fans.  It's likely your mechanical fan clutch is bad and you're
not pulling enough air to cool things down when you're not moving.  Also,
check to see if your electric aux fan is coming on when the engine is
running hot.

Neither fan is necessary when you're at speed on the motorway because
there's enough air being pushed through the radiator by the motion of the
vehicle.  They're critical in stop and go traffic, however.

Regards