Like I mentioned in a recent thread, Simplex just did not use plastic properly, as in not in the right areas of the derailleurs. Why they used it on high stress areas is puzzling as it not only negatively affects the reliability/life of the derailleur, but the resulting flex also affected their performance on the bike....
I just cringed when you mentioned that you had problems making that front derailleur stay put on the seat tube. as I suspect that it's one of those with a clamp that is half plastic which most people had cracking apart on them after they also tightened it as much as they can on their bikes just to make it stay put on the seat tube....
Do yourself a favor and get the later Simplex SX 610 and the all metal front derailleur that was usually matched up to it. There's sitll a couple of Delrin parts on the SX610, but Simplex figured out how to use the material properly on the derailleur by that time.

Chombi