I would say either some sort of malware/virus sucking up resources or the PSU isn't providing enough juice at peak loads, but is stable enough for the machine not to crash. Meaning the hardware is down clocking itself.
One RAM slot going back could be indicative of flaky power.
I have found that the results you see from 3dmark's assessment of your computer's performance is extremely optimistic and often inaccurate.
Here are a few reasons it will show your score lower than it thinks it should score:
1) Most of the scores people get won't go into their database because it will not accept a score from the most recent couple drivers for your card. This means about the only people you are compared with are those who are trying to get extremely high scores through overclocking. Otherwise they wouldn't normally sabotage performance by using old drivers.
2) I have noticed that when they compare SLI and crossfire results (perhaps all results), they compare it to tri and quad fire setups. I recently tested my system with 3dmark11, and found that my system was rated as "average" when it had some extreme overclocking. When I compared to systems near my score, they were all tri-fire systems, and I have a normal crossfire system. When I used what I'd still consider a big overclock, but much less extreme, it told me my scores were low compared to the average system similar to mine.
I'd ignore what they say, it's most likely a bad comparison and compared to those with extreme overclocking, which in their databases is the average.