what are some causes for dogs to have random behaviour spells where they tremble as if terrified and lose interest in food? These spells are sporadic and can last anywhere from 20 min to 3 hours.

Have you been in contact with a vet. Your dog could be having seizures.

Here's how it works online with humans and pets in distress:
YOU tell US every single detail, starting with age, breed, and history.
You don't make volunteers give you a list of all 243 conditions that fit your brief description.
Sorry to be harsh, but it's because I care about your dog.

what are some causes for dogs to have random behaviour spells where they tremble as if terrified and lose interest in food? These spells are sporadic and can last anywhere from 20 min to 3 hours.
Is your dog a rescue and how long has he/she been in your household and how dogs do you have ?

what are some causes for dogs to have random behaviour spells where they tremble as if terrified and lose interest in food? These spells are sporadic and can last anywhere from 20 min to 3 hours.

A few questions that can help us answer you.

1. How long has this been going on? Has the dog always been this way? If so, for how long?

2. How old is the dog?

3. Is the dog a rescue?

4. Any changes in the home?

5. Any traumatic things happen before this behavior started?

6. What tests has the vet done, and what advice has the vet given? What has the vet ruled out?

If it wasn't only when he ate, and didn't last from 20 minutes to 3 hours, then I'd say seizures. But seizures don't last for 3 hours.

Answer each of my questions with as much detail as you can give, and we'll go from there. But, having said that, there are no vets on this site, and no vet can diagnose online. So if you haven't already, you need to take your dog to the vet and get a full workup done.

Another add.

When you go to your vet, have a dental exam done.