Next time you're stuck in traffic and the needle starts climbing, rev the engine to 2000 rpm and hold it there for a few seconds, if the needle starts dropping your fan clutch is OK and the radiator is working.

If the needle doesn't drop rapidly, either your fan clutch is bad and the fan isn't drawing any air or your radiator may be blocked internally.

To check the radiator, take the car out for a run to get it up to operating temperature, switch the engine off and check the radiator core by putting your hand down the back by the fan. If its hot at the top and cold at the bottom, or vice versa, the core is blocked and you'll need to replace the radiator. Or get it flushed and hope it works.

You may also have a faulty thermostat valve, whch stops the coolant circulating. Plenty of info here on this if you do a search, but you basically have to remove the thermostat and boil it in a saucepan of water to see if it opens or not.

In my opinion, most likely cause of this overheating is air in the system. Have you recently changed coolant or drained the system and topped up? If so, you may well have an air pocket in the cylinder head. You need to undo the bleed bolt in the side of the head and add more water until it comes out of the bleed hole. It's actually easier to remove the temperature sensor as the bleed bolt is kinda fiddly and its easy to strip the threads.

good luck

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1987 300e manual 250,000 km (sold)