All I can tell you is that I didn't have this problem - so it may be something in your wiring/battery etc.. but, change of circumstance, I can't go out and swap notes - Oh please, let me have my 2CV back!
I wired direct to the battery with some of that 70amp, amp wire that I happened to have left over from wiring my motorhome, I don't know if that made a difference. My chief problem was radio reception, I never stopped engine whine. I never got round to making the speakers and installation look neat, either.
I realise that you are not doing this at the moment, but I think that you might be in trouble fitting a proper auto-amp, I used the 5w amp stage of an old Blaupunkt car radio.
I don't know how much you're in to 2cv's but you may know that Citroen produced a four wheel drive version, the Saharian - from memory, by simply putting another 2cylinder and mechanics in the boot. Maybe you should do the same, but just to power the Empeg!
You may wish to bear
this in mind, and in fact, it may be the answer to your problem, most 2cv problems seem to be distributor/electrical, in my experience - well, apart from rust.