my tigra gets kind of ove rheated?

hi,
I have got over heating problem with my tigra ( in trafic) when I am driving temp is about 95 deg, but when I get in to the trafic the temp goes up to just before 100 deg, and then the rad fan kicks in but the fan dosen't spin as fast as supose to be!
it bring the temp down to about 95.
my question is, that why the fan kicks in late and also why it doesn't spin fast enogh??
regards, farzin