Francis, you have gotten some good advice so I do not want to dwell on the thermostat. When I looked at my thermostat when I bought my 300 SD, I found that someone put a piece of wood in there and there was no thermostat! That is not expensive to fix.
Also, a no brainer is go to the MegaMall and get some "PEAK" antifreeze and mix 50-50% mixture. Do not use just plain water.
Also when in traffic, avoid staying too close to the car/jeepney ahead of you. The hot gas will just add to the over heating. This is of course after you have heeded the above advice.
The main keys are
1)Antifreeze or "Coolant" in the Phils
I would check the above for your car in that order. You are bound to find something. It may also helpful to flush the radiator if at the end nothing is working to improve the situation.
'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)