I advise you to remove the top coat, because the part which is not bubbling for now will eventually be bubbling and will create an uneven painting apperance, as well as will require you extra work. If you will repaint your exterior walls, use latex paint with 100 percent acrylic resins for longer flexibility. Apply paint first on the shady side of the house and not on the sun-baked surface.

What do you suspect causes the bubbles to form?

moisture or some other contamination, painting in direct sunlight maybe

If it feels "powdery" underneath, the old paint was chalky and needed to be scrubbed off. Pressure washing is good for getting dirt off, but paint continues to "chalk" after washing.

When you paint, always work on the shady side ... tricky for a south-facing wall, but you can do it early in the morning.

Should I use a primer or will latex acrylic be enough?

When you say exterior wall, what does that mean? Is it wood siding, stucco, etc? The reason paint peels sometimes on the sunny sides is because the sun is pulling moisture out and if it can't get out, it goes through the paint layer, causing it to bubble or fail everywhere. If this is the reason, then a primer can sometimes be a bad idea. Of course, it the paint is peeling because of poor prep or painting over chalk and dirt, then it just needs to be cleaned or prepared better.