What are the odds 3 of 4 RAM cards go bad around the same time?

For a while, I was having lots of problems with my computer that seemed to come out of nowhere. Lots of BSODs, occasional freezing (not during any intensive mind you), occasional crashes, and some weird issues with AVG not updating (general error). My computer came with 3 GB of RAM: 1 GB x 2 and 512 MB x 2. I tried taking all but one 1 GB card out and ALL the issues went away. I only noticed that it was slower during startups and shutdowns but otherwise about the same as before these issues arose. I let it run on this one card for a day (PLENTY of time for my issues to come up) and it was fine. I tried adding the other 1 GB the next day and instant BSOD so I took it out. I tried one 512 MB and my machine wouldn't turn on, I just got a black screen. I checked to make sure it was in properly, black screen is all I would get. I'm now running with the one 1 GB on the other 512 MB and it seemed fine until I had a random crash and now AVG is giving me that error again. It seems like all but the first 1 GB card went bad all at once. How likely is this and what could have caused it? Could it be an issue with the slots rather than the actual cards?