The artic cooler is not the best cooler, but in *** same breath its not the worst either.
I have a Q6600 and in the DS3 ( Oh, now Im using the DS4, so I will go to basics firstly )
I got the DS4 only the other day, so I ran it at stock for a couple of hours, gave it a fresh install, went for Vista64 as I want to play with DX10 for a bit before going to a real os ( XP64 ) again.
Anyway, I went into the BIOS, set the volts to 1.4 ( Might be more than necessary, but I ran it there on the DS3 just fine ) and the Multi, I left alone, its at 9 and the FSB set to 333 ( gives 3Ghz of course ) and thats a nice round figure and ran it for a day and it was fine.
Me personally have never really taken prime too literally... too many people will **** you off if its not prime stable, and yonks ago when I started out in clocking, I had a 100% perfectly stable system, hitting 3Ghz with an XP17, it ran absolutely everything I wanted it to and it never crashed once, but it had issues about 8 hours into prime testing, and just because of that, I had people tellign me that its not stable??? WTF? - in the end I got fed up and told them to go... Well, I better not Id get banned.
Anyway, it ran great for a day, and I upped it to 400 x 9 and the volts are 1.4v and again, its stable enough. I have not had any issues yet, and its been about 4 days now. Im folding on all 4 cores, and I am able to run 4 copies of ConvertXtoDVD v2 as well as burn 3 DVDs ( 2 speed ) and play UnReal Tournament III all at the same time and I have not had any issues at all, so as far as I am concerned... its stable.
Sure, I think that the NB could do with a notch as could the RAM, but I wont do that unless I had a cause to.
You say that your PC is unstable at 375, so check the RAM volts first, but also you say that the CPU is at stock too, so my first thought would be the CPU volts needing a small nudge in the upwards direction first... 1.4v is more than enough so, try 1.35 or 1.375 ( Some boards allow really fine tuning of the volts but I feel that to be a little bit of a waste of time to be honest, so just go in.25v steps and you will be fine.
And CPU-Z has been mentioned more than once so its good advise and a great tool to use.
Your Temps and Volts can also be read with SpeedFan too!