Do all fish float when they die?

Why does my fish float on it's side?
Every now and then one of my fish floats on it's side as though it is dead. But after a few minutes or a nudge by one of the others it starts moving again. It swims funny for a second or two as though slightly stiff then returns to normal. This is happening more often now but used to be once a week at first.

Is he having strokes or some kind of fits and is there anything we can do?

Genuine, helpful answers preferred!

Additional Details
Just to add, the fish is around 7 or 8 years old and shares a large tank with 3 other fish, He has being doing this for the last month or so but it has become more frequent. The other fish appear fine.

it sounds like its got a swim bladder problem,which can be caused by constipation from over feeding don't feed it for 3 days and then feed shelled peas for another 3 days
just because a fish has a swim bladder prob it does not mean its going to die don't answer if you don't know the answer