diagnostic help

Over the past 2 years I've had 9 ex free-range hybrids for eggs. With the exception of 2 of them, the rest have died showing the same symptoms. They each have died, one at a time,usually 3-4 months apart whilst the remainder appear healthy. Could you please give me your opinions on what is killing them. Their symptoms are (in this order) :

• eat more
  become withdrawn
  less active
  rapid weight loss to just skin and bone
   fluff-up with head tucked in feathers
  watery poo (have seen worms)
  
  fall over when moving about
  death.
  
  This whole process occurs over about 2 weeks
  There are no chicken friendly vets in my area so I have to figure this out for myself or with your help.
  I think it's worms but did not think that was fatal. Each time it happens I worm the flock again. However I could be reading the dosage incorrectly as it is intended for commercial-sized flocks.
  

I'm using Aviverm which is all I can find here.The directions state to use 1ml/9Kg live weight into 8 hours water usage. The birds don't like it and prefer not to drink it so I've been mixing their mash with it. Lately I've put a little honey or molasses to sweeten it but the change of colour (molasses) also put them off. The bird that prompted this just died so down to one chicken now. I'd like to know what's killing them before I get more replacements.

hopefully this link will take you to a photo that will show you just why worms can be fatal:-

http://poultrykeeper.com/common-articles-to-all-poultry/health/poultry-wormers.html

you say your birds are ex free rangers - so how old are they? Most egg laying hybrids will die at around the age of 3 (there are a few exceptions and I have a few of them  ).

Further I received these birds as adults, so have no idea how old they were.