Mine had exactly this problem after 2 years with the RIGHT (NON_RESISTOR - see item 2 below) plugs and 18 months on a 'new' rotor arm.

For the price of a rotor arm (I got my MB original for £30), I'd recommend trying that. I say this for seveal reasons:

1. On my 104 engine, the distributor is at the top of the engine, bolted close to the hot bits and it does get hot.

2. The rotor arm contains a 1K ohm resistor and this can break down under heat, giving exactly these symptoms. The suppression resistor is here and if you add more resistance at the plugs, misery is the only result.

3. My cheap Bremi rotor arm died after about 18 months.

4. Er, I can't remember the 4th reason.

>>> I've just remembered - I had a leaky EHA (no, the EHA, not just the grommets) and I replaced it with an old one I recovered from a 1987 4-Matic and all's been well ever since in that respect.

So, to recap, correct plugs and original quality distributor parts in that hot and hostile area are a good starting point before worrying about expensive things.

Bonne chance.

RayH