this topic has been covered numerous times, use the search feature in this site http://contour.org/ceg-vb/search.php
even if yours sits on L, the temperature might not be exactly right. Bust out the thermometer and get couple of readings. most of the time coolant temperature is fine.
I bet this only happens to you in city traffic, and slow highways; been there
if its hanging in the "A"/"L" range then replace the sending unit. its on the under side of the thermostat housing. its very common for it to go bad.Thats what I would say.
Also I will say that when I replaced my water pump and flushed the cooling system really good, my temp gauge reads right at the middle most of the time. But it still jumps in the "L" range if I'm in traffic, but not for long.