I got her from the same place as Tia from a farm in Carmarthen, they are half sisters - same fathers - different mothers.
I saw both mothers each time but the dad was out on the farm. ( working farm not puppy one).
Sadly, from reading the other thread in genetics and breeding, I DO think this place was a puppy farm. Unfortunately Camarthen is an area with several puppy farms...they won't always be instantly recognisable as puppy farms..but they are.
I really think your next port of call as Penny says is to exclude physical reasons for the behaviour. If she is given a clean bill of health by the vet, a consultation with a behaviourist would be the best idea. Your vet might have the details of a behaviourist. But as Penny said, it would be better if possible to speak to someone with experience or knowledge of rage. Some behaviourists don't believe it exists and some blame every problem in cockers on rage, neither approaches are helpful.
I know there is a helpline for rage too, you may be about to get more advice about the dog's behaviour, BUT you really do need to get someone to see the dog first hand. She could be displaying signs of dominant behaviour that could be worked through with help.