The coolant used by GM since 1996 in all GM cars is Dexcool, its a long life
coolant and is 'supposed' to last 100,000 miles or 5 years before it needs
to be changed. If the coolant is brown it should be changed and new Dexcool
put in. Dexcool is an orange color. Make sure that the service facility you
take the car to flushes the cooling system  really well and the surge tank
cleaned out.
Your temperature seems normal.
I believe that if you read the Haynes manual again you will see that it does
make reference to Dexcool --Page 1-13 section 10.
Hope this helps. My 2 cents for what its worth.

Hi,

I have very litle knowledge on cars, but have a 99 Malibu. Brought it 1
year ago and had no problems.

I noticed that temperature is usualy one line above the middle of the
scale, and when I'm stuck in traffic and drive slow (10km/h) the
temperature will go 3 line above middle, then I need to put heating to
maximum and start ventilators to cool it down 2 lines, but this sometimes
doesn't work.

The liquid inside container is brown in color. Is this normal? I have
133.000 km now.

I brought Haynes manual and they say that the coolant should be green in
color, so I'm thinking on flushing the whole system and replace the
coolant.

What do you say?

Thank you,
Cris

Flush & fill immediately. Could be rust in your radiator passages. Also,
while some recommend the orange stuff.. I've heard nothing but bad things
about it. I'd stick w/ the green stuff if it were my car... but it isn't..
so do what you feel like.

-Mike

--
A happy kid behind the wheel of a 98 Mustang GT
Cold air intake
FRPP 3.73 gears
Steeda Tri-Ax Shifter
Flowmaster 40 Series mufflers (self-installed woohoo)
Hi-speed fan switch
255/60R-15 rear tires
Subframe connectors