I have an 11 month old staff, I love him dearly, but Im seriously worried about his recent behaviour. I also have an 11 month old cat, who I secretly favour, cats are my true love 
To start with puppy and kitten were great together, odd bit of chasing each other but it was all gentle an playful, thu used to curl up together on te sofa etc, all seeme to be going well, until about a month ago when suddenly dog stepped up the pace. He's now seeking the cat out and snapping at him, aggressively chases him, tries to pin him down, tries to mouth him iykwim. Tonight he has cleared the back fence tryin to catch the cat hi was sitting minding his own business and my cat is terrified. I love my dog but in all honesty if he hurts my cat I'll kill him!
Also Dog seems to be suffering from separation anxiety perhaps, or he's just a shite bag! For example, if I am talking at the for to postie for a minute and won't let him past me/with me he will go and wee somewhere/chew something up, then I see it and he slinks off before I even have chance to react! If he is left in his cage - over night or when we go out he is fine, but I don't have time to put him in there before I answer the door for example or taking dd (potty training) to the loo! It's driving me a bit nuts tbh. I am a sahm so about all most a the time with him, and the back door is always open for him.
Can anyone give me some advice on either Of these issues?
Must add he's bloomin beautiful and great with the kids 