Obvious but this is just for clarification so stay with me on this.
You need to rule out the monitor (Try something else in it or the mega on another screen)
Run a memory check program to rule out the Marpet upgrade and the Ram in it (Although you have replaced the ram the board might be faulty and might have dry joints on it).
If this is all fine then it must be the mainboard but if the lines don't move then it is more likely to be a RAM error.
If it is the RAM check the board for bent pins in the SIMM sockets and dry joints on all the boards. also clean the pins on the boards.
RE: MEGA ST vertical lines on screen 
