Thanks everyone, I will call my vet and find out if they can do a necropsy, or get them to find out where I can get one done. I think its going to be the only way I can find out whats going on. I can pretty much rule out poisoning as the only place they visit occasionally is next doors paddock that my neighbors use as a dog agility training area, I know they don't put anything down in there. They do wander onto our lane at times, but nobody sprays around here. I can't think they are drinking antifreeze (certainly not on our place) and the feed isn't moldy as its kept in its sack in a bin in the barn and my gang go through about 2 sacks of feed a week! Whatever is affecting the guineas isn't affecting the chickens.......................and they share the same coop.
Feed can come to you with mold in it direct from the store. I've had it happen and I didn't realize it until most was eaten and two hens were dead from crop stasis-nothing appeared moldy until the bottom of the bag.
Contact a State Poultry Lab. The state vet would be the one to do the necropsy. Could even be free or very reasonable, more so than an actual vet.