I am using q9400 with intel dg41pr board on stock speed, I have 2 GB ram and 160+500 sata hdd. i bought this proc and mobo 2-3 weeks back. In the first week all was working perfect then suddenly my PC started rebooting while rendering some 3D scenes in 3DS max or starting the prime95 test. I monitor my CPU temperature with hwmonitor. It was around 45-50 on idle and around 72+ on load. then i opened my case and pulled out my processor and replaced it with my old e4500 and it started working fine. with idle temps of 38 and on load 56-60. then i noticed the rpm of cpu fan, it was around 3200rpm on load and around 1700 on idle. I put back my q9400 again in the mobo and it starting working fine with load temp of max 62 and 3200 rpm. It worked fine for a week but now it again giving the same problem. as I start a heavy application it restarts. but the windows 7 is not giving me any message. its idle temp is 52 with 1700rpm while my room temp is around 32 and on load it goes to 77+ but rpm goes to maximum 2200.
my observation is: this is happening due to the slow speed of cpu fan, but question is why its not running according to requirements. Let my also tell you i am not using the stock hsf. I replaced it with Pentium D hsf which was far heavier then the stock q9400 hsf.
go in your bios and check to see if cpu fan speed is at 100 when the cpu tempurter reaches aroun 50C. if that doesnt fix the problem try doing a virus scan check, disk clean up, and disk defrag. Try to clean the vents on the computer case
From my experience with the q9400, it tends to run a bit hot at times; no doubt about that. Using the stock heat sink that came with it I was seeing temps like you were seeing. So, I bought an after market cooler (SpinQ) and lapped it. With the SpinQ on high and the lap job, most of the cores idle around 32C and none of the cores even approaches 55C using Intel Burn Test (no OC).
I'd recommend investing in an after market cooler/decent thermal paste and ditching the Pentium D cooler, as I believe that is most of the problem. Remember, you're dealing with 4 cores- not 2. Don't worry about lapping it or anything like that- I did it for the heck of it and I've had tons of practice. All in all, with a better cooler, you'll be able to adjust the fan speed at will and keep it nice and cool.