The problem sound much like a reseating problem. Part of incorrectly installing a ram upgrade causes the MMU socket pins to be squashed. This can cause a varitey of results from blank screens to all manner of effects and colours. To rectify the issue you need to first remove the chip from the socket. reseating won't do here you need to apply some gentle pressure to each pin in order to force the pin to close the gap slightly thus ensuring a good connection. Be patcient as the first time you might not get them all right and will need to repeat the process.
