Prime has never been shaky for me.
Running other programs at the same time like 3dmark loops or memtest will interupt the execution of the prime code, and give your hardware intermittent breathers - they should be run seperately. You should run prime by itself at high priority so there are no services or processes to interupt it.
Stable is more than 12 hours of prime without an error IMO, and that is what most people say who are serious about stability and running GOOD overclocks - if you run it long and hard enough, the chance of crashing out of a program or getting data errors can be forgotten about.
If you get an error in prime after 5 hours or even 10 hours, that means you've got a problem somewhere. This problem you've found could possibly pop up an hour into playing a game, 2 hours into encoding that video, or a few hours after folding. If you get an error while your testing prime, then you can experience a random error caused by system instability at any point while you are using your computer.
If you get an error while you are running prime in normal priority, that is even a worse sign... if it takes you 5 hours to get an error with prime running normal priority, you likely would have found it sooner if you ran it at high priority.
If an occasional random crash, error, or lockup is okay with you, then you don't need to be so obsessive about testing.
