Sorry for my late response. I'm Swedish.
I remember getting an income letter from my embassy (for a different purpose) a couple of years ago, but I'm not sure that's going to help now.
Also, because the money is not from any kind of pension fund (and the amont varies), won't I basically incriminate myself as someone working without a work permit?
I'd be interested to know if anyone has had problems getting multiple consecutive Single Entry visas, as this seems to be my only recourse for the time being.