Bad memory/RAM chips can result in problems like distorted graphics or text. Figuring out which chip(s) needs to be replaced can be done easily with a logic probe. Set the probe to pulse and TTL/LS. Looking at the motherboard (with the cart port towards you), the memory chips are in the upper-right area and are labeled U10-U17. With the system on, probe the pins on each chip. Here's what you should be seeing:

1) LOW
2) PULSE
3) HIGH
4) PULSE
5) PULSE
6) PULSE
7) PULSE
8) HIGH
9) HIGH
10) PULSE
11) PULSE
12) PULSE
13) PULSE
14) PULSE
15) PULSE
16) LOW

If a pin gives you a different reading than what's listed, the chip is bad and needs to be replaced.