Hello and welcome to the forum. You are right in that Canagan is a good food. The only problem is that there is so much in the product that your dog could be sensitive to, peas being one of them. That being said, it doesn't necessarily mean that it is the right food for all dogs. Some dogs do better on food that is not rated so highly.
I was going to ask if your dog was a scavenger because when I read your post and the history last night it was the first thing that came to mind. The fact that this diarrhoea is intermittent seems to point to it. Coprophagia is a horrible problem and it is unlikely that you will find a cure for it.
It may be useful to tackle this methodically:
* Continue with the Canagan and give nothing else - no family food or treats. Sea jerky should be OK.
* Try to stop the scavenging. Clear up poo as soon as he has done it. Consider use of a basket muzzle and put it on when he is in the garden or out on walks. Alternatively you can watch him closely to ensure he doesn't clean up.
* Monitor for a few weeks and if the problem persists, then look at the diet.
As David has said, occasionally dogs have chicken intolerance and it is necessary to look for a poultry free food but it is difficult to say if this is the case with your dog. It would do no harm to have a word with the vet tomorrow but if you do, please take the dry analysis of the food he is on. This can be found here
on the dials near the bottom of the page. If you feel that you would like to have his poo tested for infection then take a sample in a small clean container.