Why do my pancakes fizz?
I don't have much experience of cooking with gluten free flour so can't comment on any degree to which that might be a factor. I would generally let a pancake batter sit for at least 15 minutes before use to let the flour fully absorb the liquid, that may be less necessary with gluten free flour.

However there are three main things that leap out at me from the description as being possible sources of the problem:


The recipe: pancake batter is quite thin normally, if you push the balance too far towards liquids, the batter can't form a cohesive pancake because the flour mix is too dilute, the bonds can't be formed. It will eventually form a solid, but not until heat has driven off the excess liquid. Your recipe already has a higher proportion of liquid than most UK recipes I've seen, and also less egg for the amount of batter than many. Obviously it works for you some of the time, which leads us to the next point...
Not measuring flour: Your recipe is for a pretty thin batter, that reduces the tolerance within which under-measuring still lets the batter function. I suspect that the times you see what you describe as 'separation' is when there just isn't enough flour to thicken the liquid.
Pan temperature. What you describe as 'fizzing' may be what others would just call sizzling, the mixture hit the hot pan surface and the liquid in it quickly turns to steam, causing the bubbles. This is going to be more pronounce the higher the proportion of liquid in your batter.


A minor thing is that in my experience some thing
s work better if you just whisk them together rather than using the chopping motion of the blades in a blender. I'd put pancake batter in that category, but that may be just personal preference.

I recommend that 


you start measuring the flour to ensure that you achieve a consistent
batter
Use a small amount of batter to test the temperature of the pan before you start making the pancakes proper.
Try whisking rather than blending and resting the batter, though I think these are less likely to make a difference. 


My personal preference would always be to cook pancakes in a buttered pan too. One of the benefits of that, apart from the flavour, is that because butter burns if the pan is too hot, you have to keep the pan temperature lower and that is generally a good thing for pancakes.