hello guys. i have another prob with my s2

the fan seems to have a mind of its own, sometimes it cuts in fine, but other

time it just wont cut in, for example i was stuck in traffic today and the temp got close to the red, fan wasnt on, but other times it cuts as soon as the needle hits halfway, i have changed the fan switch. what else could it be?
pls help,, gettin a bit fed up of the car now

Amit