The green screen usually appears when one of the chips of the chip ram is faulty. If you can verify that in your miggy, then you will need to find new ram chips to replace. A interesting source of this kind of chips is the old isa pc video cards. These usually have 1mb, and you could replace them. I heard of a guy that upgraded his 500 with 1mb chip, just adding 512k from one of these cards to the empty sockets of the a500 and adding the needed capacitors.
Hope this help.
