Solved! Had all the same problems. The display would be covered with checkerboard test patterns or the computer would beep on boot indicating bad memory. Other times it would boot fine, then freeze up. I even found that if it did boot without beeping and displayed a normal screen, if I squeezed the left front of the bottom case just to the left of the track pad, the screen would show the same test pattern blocks and then hang.
So I followed what was tried above and found that by folding up a 3x3 sticky pad and trimming it to the proper width (same as the memory mod.) I was able to press it between the back edge of the bottom memory module thus forcing it forward into the memory slot. Now my machine boots every time with no beep, no more artifacts on the display and I can squeeze it as hard as I can without duplicating the previous display issues.
It all comes down to a weakened bottom memory module retention clip. By pressing something, anything non-conductive at the back of the simm module to keep it pressed forward in the slot all is good again.